The Human Endocrine System

Introduction

DEFINITIONS

ENDO = internal

CRINE = secrete

a. The endocrine glands are glands of internal secretion (rather than external, as seen with the sweat glands and digestive glands).

b. This internal secretion results from the fact that these glands have no ducts. Thus, they are often referred to as the ductless glands.

c. The secretions produced by the endocrine glands are called hormones.

d. Hormones are carried by the bloodstream to specific organs or tissues, which are then called the target organs.

e. The activity of the target organ, in turn, affects the activity of the endocrine organ. Thus, it is a reverse or feedback mechanism.

GENERAL

a. Control "Systems" of the Human Body. The structure and function of the human body is controlled and organized by several different "systems."

(1) Heredity/environment. The interaction of heredity and environment is the fundamental control "system." Genes determine the range of potentiality and environment develops it. For example, good nutrition will allow a person to attain his full body height and weight within the limits of his genetic determination. Genetics is the study of heredity.

(2) Hormones. The hormones of the endocrine system serve to control the tissues and organs in general. (Vitamins have a similar role.) Both hormones and vitamins are chemical substances required only in small quantities.

(3) Nervous system. More precise and immediate control of the structures of the body is carried out by the nervous system.

b. The Endocrine System. In the human body, the endocrine system consists of a number of ductless glands producing their specific hormones. Because these hormones are carried to their target organs by the bloodstream, the endocrine organs (glands) are richly supplied with blood vessels.

c. Better Known Endocrine Organs of Humans. The better known endocrine organs are the:

(1) Pituitary body.

(2) Thyroid gland.

(3) Parathyroid glands.

(4) Pancreatic islets (islands of Langerhans).

(5) Suprarenal (adrenal) glands.

(6) Gonads (female--ovaries; male--testes).

In addition, there are several other endocrine organs, less well understood, and other organs suspected to be of the endocrine type. See figure 10-1, which shows the better known endocrine glands and their locations.

                                                                The Pituitary Body

 GENERAL

a. Location. The pituitary body is a small pea-sized and pea- shaped structure. It is attached to the base of the brain in the region of the hypothalamus (see para 11-9). In addition, it is housed within a hollow of the bony floor of the cranial cavity. This hollow is called the sella turcica ("Turk's saddle").

b. Major Subdivisions. The pituitary body is actually two glands-- the posterior pituitary gland and the anterior pituitary gland. Initially separate, these glands join together during development of the embryo.

POSTERIOR PITUITARY GLAND

The posterior pituitary gland is the portion which comes from and retains a direct connection with the base of the brain. The hormones of the posterior pituitary gland are actually produced in the hypothalamus of the brain. From the hypothalamus, the hormones are delivered to the posterior pituitary gland, where they are released into the bloodstream. At present, we recognize two hormones of the posterior pituitary gland.

a. ADH (Antidiuretic Hormone). ADH is involved with the resorption or salvaging of water within the kidneys. ADH is produced under thirst conditions.

b. Oxytocin. Oxytocin is concerned with contractions of smooth muscle in the uterus and with milk secretion.

ANTERIOR PITUITARY GLAND

a. The anterior pituitary gland originates from the roof of the embryo's mouth. It then "attaches" itself to the posterior pituitary gland.

b. The anterior pituitary gland is indirectly connected to the hypothalamus by means of a venous portal system. By "portal," we mean that the veins carry substances from the capillaries at one point to the capillaries at another point (hypothalamus to the anterior pituitary gland).

c. In the hypothalamus, certain chemicals known as releasing factors are produced. These are carried by the portal system to the anterior pituitary gland. Here, they stimulate the cells of the anterior pituitary gland to secrete their specific hormones.

d. The anterior pituitary gland produces many hormones. In general, they stimulate the target organs to develop or produce their own products. This stimulating effect is referred to as trophic.

e. Of the many hormones produced by the anterior pituitary gland, we will examine:

(1) Somatotrophic hormone (growth hormone). The target organs of this hormone are the growing structures of the body. This hormone influences such structures to grow.

(2) ACTH (adrenocorticotrophic hormone). This hormone of the anterior pituitary gland stimulates the cortex of the suprarenal (adrenal) gland to produce its hormones. We will later see that the hormones of the suprarenal cortex are involved with anti-inflammatory reactions of the body.

(3) Thyrotropin (TSH). This hormone stimulates the thyroid gland to produce its hormones.

(4) Luteinizing hormone (LH). LH stimulates ovulation and luteinization of ovarian follicles in females and promotes testosterone production in males.

(5) Follicle-stimulating hormone (FSH). FSH stimulates ovarian follicle growth in females and stimulates spermatogenesis in males. (6) Prolactin. Prolactin stimulates milk production and maternal behavior in females.

                                                                The Thyroid Body
LOCATION

The thyroid gland is in the neck region just below the larynx and surrounds the trachea.

ANATOMY

a. The right and left thyroid lobes are the masses on either side of the trachea. The isthmus is found across the front of the trachea and connects the two lobes.

b. Each lobe of the thyroid gland is supplied by arteries from above and below (superior and inferior thyroid arteries).

HORMONES

The primary hormone of the thyroid gland is thyroxin. Thyroxin affects the basal metabolic rate (BMR), the level of activity of the body. Since iodine is a necessary element in the production of thyroxin, one can observe malformations of the thyroid gland (called goiters) where there is little or no iodine available. A second hormone, calcitonin, is produced by the thyroid gland and it is involved with calcium metabolism in the body.

                                                             The Parathyroid Glands 


 LOCATION AND STRUCTURE

Located on the posterior aspects of the thyroid lobes are two pairs of small round masses of tissue, known as the parathyroid glands.

HORMONE

The hormone produced by these glands is called parathyroid hormone, or parathormone. It is involved with calcium metabolism.

                                             The Pancreatic Islets (Islands of Langerhans) 

LOCATION AND STRUCTURE

Within the substance of the pancreas are distributed small groups of cells known as islets. Although the pancreas is a ducted gland of the digestive system, these isolated islets are, in fact, ductless glands.

HORMONES

Insulin and glucagon are the two most commonly recognized hormones of the islets. These hormones are involved with glucose metabolism.

                                                          The Suprarenal (Adrenal) Glands 

LOCATION AND STRUCTURE

Embedded in the fat above each kidney is a suprarenal gland. Both suprarenal glands have an internal medulla and an external cortex.

HORMONES OF THE SUPRARENAL MEDULLA

The medullary portion of each suprarenal gland produces a pair of hormones--epinephrine (adrenalin) and norepinephrine (noradrenalin). These hormones are involved in the mobilization of energy during the stress reaction ("fight or flight").

HORMONES OF THE SUPRARENAL CORTEX

Each suprarenal cortex produces a variety of hormones which can be grouped into three categories:

a. Mineralocorticoids (for example, aldosterone), which are concerned with the electrolytes of the body.

b. Glucocorticoids (for example, cortisol), which are concerned with many metabolic functions and are anti-inflammatory in nature.

c. Sex hormones. Adrenal androgens and estrogens.

                                                                    The Gonads

 GENERAL

In humans, the primary sex organs are known as gonads (lesson 8). The gonads produce sex cells (gametes) and sex hormones. These sex hormones are in addition to those produced by the suprarenal cortex

FEMALE SEX HORMONES

In the female, the ovaries produce two types of sex hormones during the menstrual cycle. During the first half of the cycle (days 1 - 14), the estrogens are produced. During the last half of the cycle (days 15 - 28), progesterone is produced. These hormones are concerned with female sexuality and with the preparation of female sex organs for reproduction.

MALE SEX HORMONES

In the male, certain cells of the testes produce the male sex hormones known as androgens (for example, testosterone). Androgens are concerned with male sexuality.

The Human Cardiovascular and Lymphatic Systems

Introduction

NEED FOR CIRCULATORY SYSTEMS

a. The need for circulatory systems is based on two criteria:

(1) Number of cells. Multicellular animals are animals with a great number of cells.

(2) Size. In larger animals, most cells are too far away from sources of food and oxygen for simple diffusion to provide sufficient amounts. Also, distances are too great for simple removal of wastes.

b. Because of these criteria, we need a system (or systems) to carry materials to all cells. To get food and oxygen to the cells and to remove waste products, we need a transport system or circulatory system. Human circulatory systems are so effective that no cell is more than two cells away from a blood capillary.

BASIC COMPONENTS OF ANY CIRCULATORY SYSTEM

The four basic components of any circulatory system are a vehicle, conduits, a motive force, and exchange areas.

a. Vehicle. The vehicle is the substance which actually carries the materials being transported.

b. Conduits. A conduit is a channel, pipe, or tube through which a vehicle travels.

c. Motive Force. If we say that a force is motive, we mean that it produces movement. Systems providing a motive force are often known as pumps.

d. Exchange Areas. Since the materials being transported must eventually be exchanged with a part of the body, special areas are developed for this purpose. They are called exchange areas.

CIRCULATORY SYSTEMS IN THE HUMAN BODY

a. The cardiovascular system is the circulatory system involving the heart and blood vessels.

b. The lymphatic system is a drainage-type circulatory system involved with the clear fluid known as lymph.

c. There are other minor circulatory systems in the human body, such as the one involved with cerebrospinal fluid.

The Human Cardiovascular System

 GENERAL

The human cardiovascular system is a collection of interacting structures designed to supply oxygen and nutrients to living cells and to remove carbon dioxide and other wastes. Its major components are the:

a. Blood. Blood is the vehicle for oxygen, nutrients, and wastes.

b. Blood Vessels. Blood vessels are the conduits, or channels, through which the blood is moved.

c. Heart. The heart is the pump which provides the primary motive force.

d. Capillaries. The capillaries, minute (very small) vessels, provide exchange areas. For example, in the capillaries of the lungs, oxygen is added and carbon dioxide is removed from the blood.

BLOOD

Blood is the vehicle for the human cardiovascular system. Its major subdivisions are the plasma, a fluid containing proteins, and the formed elements, including red blood cells, white blood cells, and platelets.

a. Plasma.

(1) Plasma makes up about 55 percent of the total blood volume. It is mainly composed of water. A variety of materials are dissolved in plasma. Among the most important of these are proteins.

(2) After the blood clots, the clear fluid remaining is called serum. Serum does not contain the proteins used for clotting. Otherwise, it is very similar to plasma.

b. Formed Elements. The formed elements make up about 45 percent of the total blood volume. The formed elements are cellular in nature. While the red blood cells (RBCs) and white blood cells (WBCs) are cells, the platelets are only fragments of cells.

(1) Red blood cells (erythrocytes). RBCs are biconcave discs. That is, they are shaped something like an inner tube from an automobile tire, but with a thin middle portion instead of a hole. There are approximately 5,000,000 RBCs in a cubic millimeter of normal adult blood. RBCs contain hemoglobin, a protein which carries most of the oxygen transported by the blood.

(2) White blood cells (leukocytes). There are various types of WBCs, but the most common are neutrophils and lymphocytes. Neutrophils phagocytize (swallow up) foreign particles and organisms and digest them. Lymphocytes produce antibodies and serve other functions in immunity. In normal adults, there are about 5,000 to 11,000 WBCs per cubic millimeter of blood.

(3) Platelets. Platelets are about half the size of erythrocytes. They are fragments of cells. Since they are fragile, they last only about three to five days. Their main function is to aid in clotting by clumping together and by releasing chemical factors related to clotting. There are 150,000 - 350,000 platelets in a cubic millimeter of normal blood.

c. Some General Functions of the Blood.

(1) Blood serves as a vehicle for oxygen, nutrients, carbon dioxide and other wastes, hormones, antibodies, heat, etc.

(2) Blood aids in temperature control. Beneath the skin, there is a network of vessels that functions much like a radiator. To avoid accumulation of excess heat in the body, the flow of blood to these vessels can be increased greatly. Here, aided by the evaporative cooling provided by the sweat glands, large amounts of heat can be rapidly given off. The flow of blood also helps keep the outer parts of the body from becoming too cold.

(3) The blood aids in protecting our bodies by providing immunity. Some WBCs phagocytize (swallow up) foreign particles and microorganisms. Other WBCs produce antibodies. The blood transports antibodies throughout the body.

(4) Blood clotting is another function of blood. Not only does this prevent continued blood loss, it also helps prevent invasion of the body by microorganisms and viruses by sealing the wound opening.

BLOOD VESSELS

The blood is conducted or carried through the body by tubular structures known as blood vessels. Since at no time does the whole blood ever leave a blood vessel of some sort, we refer to this system as a closed system.

a. General Construction. The blood vessels in general are tubular and have a three-layered wall.

(1) Intima. The lumen (hollow central cavity) is lined by a layer of smooth epithelium known as the intima.

(2) Media. A middle layer of smooth muscle tissue is called the media.

(3) Adventitia. The adventitia is the outer layer of fibrous connective tissue that holds everything together.

b. Types of Blood Vessels. See figure 9-1 for a diagram of the human circulatory system. We recognize three types of blood vessels:

(1) The arteries carry blood away from the chambers of the heart.

(2) The veins carry blood to the chambers of the heart.

(3) Capillaries are extremely thin-walled vessels having only the intimal layer through which exchanges can take place between the blood and the tissue cells.

c. Relationships. Arteries and veins are largest where they are closest to the heart. Away from the heart, they branch into smaller and smaller and more numerous vessels. The branching continues until the smallest arteries (arterioles) empty into the capillaries. The capillaries in turn are drained by the venules of the venous system.

d. Valves. Within the heart and the veins are structures known as valves. Valves function to insure that the blood flows in only one direction.

THE HEART

Through the action of its very muscular walls, the heart produces the primary motive force to drive the blood through the arterial system. In humans, the heart is located just above the diaphragm, in the middle of the thorax, and extending slightly to the left. It is said that the heart of an average individual is about the size of that individual's clenched fist.

a. General Construction of the Human Heart. See figure 9-2 for an illustration of the human heart.

(1) Chambers. The heart is divided into four cavities known as the chambers. The upper two chambers are known as the atria, right and left. Each atrium has an ear-like projection known as an auricle. The lower two chambers are known as ventricles, right and left. Between the two atria is a common wall known as the interatrial septum. Between the two ventricles is a common wall known as the interventricular septum.

ATRIUM = hall

AURICLE = ear-like flap

VENTER = belly

SEPTUM = fence

(2) Wall layers. The walls of the chambers are in three general layers. Lining the cavity of each chamber is a smooth epithelium known as the endocardium. (Endocarditis is an inflammation of the endocardium.) The middle layer is made up of cardiac muscle tissue and is known as the myocardium. The outer layer of the heart is another epithelium known as the epicardium.

(3) Relationship of wall thickness to required pressure levels. A cross-section of the chambers shows that the atrial walls are relatively thin. The right ventricular wall is much thicker. The left ventricular wall is three to five times thicker than that of the right. These differences in wall thickness reflect the amount of muscle tissue needed to produce the amount of pressure required of each chamber.

(4) Cardiac valves (figure 9-3).

(a) Between the atrium and ventricle of each side is the atrioventricular (A-V) valve. Each A-V valve prevents the blood from going back into the atrium from the ventricle of the same side. The right A-V valve is known as the tricuspid valve. The left A-V valve is known as the mitral valve. ("Might is never right.") The leaflets (flaps) of the A-V valves are prevented from being pushed back into the atria by fibrous cords. These fibrous cords are attached to the underside (the ventricular side) of the leaflets and are called chordae tendineae. At their other ends, the chordae tendineae are attached to the inner walls of the ventricles by papillary muscles.

(b) A major artery leads away from each ventricle--the pulmonary trunk from the right ventricle and the aortic arch from the left ventricle. A semilunar valve is found at the base of each of the pulmonary trunk and the aortic arch. These semilunar valves prevent blood from flowing back into the ventricles. The pulmonary (semilunar) valve and the aortic (semilunar) valve are each made up of three semilunar ("pocket-like") cusps.

b. Control of the Heart Beat. The heart is under several different control systems--extrinsic nervous control, intrinsic nervous control, and humoral control.

(1) Extrinsic nervous control. Extrinsic nervous control is control from outside of the heart. Extrinsic control is exerted by nerves of the autonomic nervous system. The sympathetic cardiac nerves accelerate (speed up) the heart. The vagus parasympathetic nerve decelerates (slows down) the heart.

(2) Intrinsic "nervous" control. Intrinsic "nervous" control is control built within the heart. The intrinsic "nervous" system consists of the sinoatrial (S-A) node (often referred to as the "pacemaker"), the atrioventricular (A-V) node, and the septal bundles. The septal bundles spread through the walls of the ventricles, just beneath the endocardium. This combination of nodes and bundles initiates the heart beat automatically and transmits the impulse through the atria and the ventricles.

(3) Humoral control. In addition to the "nervous" control of heart action, it appears that there are substances in the blood itself which have varying effects on the functioning of the heart. Although these substances are not as yet well understood, they appear to have some importance. The transplanted heart seems to depend to a degree on this control mechanism, since much of its "nervous controls" have been lost for the initial period in the recipient's body.

c. Coronary Arteries and Cardiac Veins. We may say that the heart deals with two different kinds of blood flow--"functional" blood and "nutritive" blood. "Functional" blood is the blood that the heart works on or pushes with its motive force. However, the walls of the heart require nutrition that they cannot get directly from the blood within the chambers. "Nutritive" blood is supplied to these walls by the coronary arteries, right and left. The coronary arteries arise from the base of the aortic arch and are distributed over the surface of the heart. This blood is collected by the cardiac veins and empties into the right atrium of the heart. Should a coronary artery, or one of its branches, become closed for whatever reason, that part of the heart wall formerly supplied nutrient blood by the closed vessel will very likely die.

d. Pericardial Sac. The average heart contracts in what is known as a heart beat, about 70-80 times a minute. To reduce the frictional forces that would be applied to its moving surfaces, the heart is enclosed in a special serous sac known as the pericardium ("around the heart").

CARDIOVASCULAR CIRCULATORY PATTERNS

See figure 9-4 for an illustration depicting cardiovascular circulatory patterns.

a. General. The human cardiovascular circulatory system is described as a closed, two-cycle system.

(1) It is closed because at no place is the blood as whole blood ever outside the system.

(2) It is two-cycle because the blood passes through the heart twice with each complete circuit of the body. In the pulmonary cycle, the blood passes from the right heart, through the lungs, and to the left heart. In the systemic cycle, the blood passes from the left heart, through the body in general, and returns to the right heart.

(3) It is common for an area of the body to be supplied by more than one blood vessel so that if one is damaged, the others will continue the supply. This is known as collateral circulation. However, there are situations, such as in the heart and the brain, where a single artery supplies a specific part of a structure. Such an artery is called an end artery. When an end artery is damaged, that area supplied by it will usually die, as in the case of the coronary artery (para 9-7c) above or in the case of a "stroke" in the brain.

b. Pulmonary Cycle. The pulmonary cycle begins in the right ventricle of the heart. Contraction of the right ventricular wall applies pressure to the blood. This forces the tricuspid valve closed and the closed valve prevents blood from going back into the right atrium. The pressure forces blood past the semilunar valve into the pulmonary trunk. Upon relaxation of the right ventricle, back pressure of the blood in the pulmonary trunk closes the pulmonary semilunar valve. The blood then passes into the lungs through the pulmonary arterial system. Gases are exchanged between the alveoli of the lungs and the blood in the capillaries next to the alveoli. This blood, now saturated with oxygen, is collected by the pulmonary veins and carried to the left atrium of the heart. This completes the pulmonary cycle.

c. Systemic Cycle.

(1) Left ventricle of the heart. The oxygen-saturated blood is moved from the left atrium into the left ventricle. When the left ventricular wall contracts, the pressure closes the mitral valve, which prevents blood from returning to the left atrium. The contraction of the left ventricular wall therefore forces the blood through the aortic semilunar valve into the aortic arch. Upon relaxation of the left ventricular wall, the back pressure of the aortic arch forces the aortic semilunar valve closed.

(2) Arterial distributions. The blood then passes through the various arteries to the tissues of the body. See figure 9-5 for an illustration of the main arteries of the human body.

(a) The carotid arteries supply the head. The neck and upper members are supplied by the subclavian arteries.

(b) The aortic arch continues as a large single vessel known as the aorta passing down through the trunk of the body in front of the vertebral column. It gives off branches to the trunk wall and to the contents of the trunk.

(c) At the lower end of the trunk, the aorta divides into right and left iliac arteries, supplying the pelvic region and lower members.

(3) Capillary beds of the body tissues. In the capillary beds of the tissues of the body, materials (such as food, oxygen, and waste products) are exchanged between the blood and the cells of the body.

(4) Venous tributaries. See figure 9-6 for an illustration of the main veins of the human body.

(a) The blood from the capillaries among the tissues is collected by a venous system parallel to the arteries. This system of deep veins returns the blood back to the right atrium of the heart.

(b) In the subcutaneous layer, immediately beneath the skin, is a network of superficial veins draining the skin areas. These superficial veins collect and then join the deep veins in the axillae (armpits) and the inguinal region (groin).

(c) The superior vena cava collects the blood from the head, neck, and upper members. The inferior vena cava collects the blood from the rest of the body. As the final major veins, the venae cavae empty the returned blood into the right atrium of heart.

(d) The veins are generally supplied with valves to assist in making the blood flow toward the heart. It is of some interest to note that the veins from the head do not contain valves.

(e) From that portion of the gut where materials are absorbed through the walls into the capillaries, the blood receives a great variety of substances. While most of these substances are useful, some may be harmful to the body. The blood carrying these substances is carried directly to the liver by the hepatic portal venous system. This blood is specially treated and conditioned in the liver before it is returned to the general circulation by way of the hepatic veins.

The Human Lymphatic System

 GENERAL

Between the cells of the body are spaces filled with fluid. This is the interstitial (or tissue) fluid, often referred to as intercellular fluid. There are continuous exchanges between the intracellular fluid, the interstitial fluid, and the plasma of the blood. The lymphatic system returns to the bloodstream the excess interstitial fluid, which includes proteins and fluid derived from the blood.

STRUCTURES OF THE HUMAN LYMPHATIC SYSTEM

a. Lymphatic Capillaries. Lymphatic capillaries are located in the interstitial spaces. Here, they absorb the excess fluids.

b. Lymph Vessels. A tributary system of vessels collects these excess fluids, now called lymph. Like veins, lymphatic vessels are supplied with valves to help maintain a flow of lymph in one direction only. The lymphatic vessels, to a greater or lesser extent, parallel the venous vessels along the way. The major lymph vessel in the human body is called the thoracic duct. The thoracic duct passes from the abdomen up through the thorax and into the root of the neck in front of the vertebral column. The thoracic duct there empties into the junction of the left subclavian and jugular veins.

c. Lymph Nodes. Along the way, lymphatic vessels are interrupted by special structures known as lymph nodes. These lymph nodes serve as special filters for the lymph fluid passing through.

d. Tonsils. Tonsils are special collections of lymphoid tissue, very similar to a group of lymph nodes. These are protective structures and are located primarily at the entrances of the respiratory and digestive systems.

The Human Urogenital Systems

The Human Urinary System

DEFINITION

The human urogenital systems are made up of the urinary organs, which produce the fluid called urine, and the genital, or reproductive, organs of male and female humans, which together can produce a new human being.

INTRODUCTION TO THE HUMAN URINARY SYSTEM

a. Proteins are one of the basic foodstuffs that humans consume. When proteins are used by the body, there are residue or waste products which can be poisonous (toxic) if allowed to accumulate in large amounts. The urinary system of the human body is specialized to remove these nitrogenous waste products from the circulating blood.

b. Major Parts. See figure 8-1 for the major parts of the human urinary system. This system includes two kidneys, two ureters (one connecting each kidney to the urinary bladder), the urinary bladder, and the urethra.

THE KIDNEY

a. General.

(1) The kidneys have the same shape and color as kidney beans, but are about 8-10 centimeters (3-3 1/2 inches) in length.

(2) Each kidney has a fibrous capsule. On the concave, medial side of each kidney, there is a notch called the hilus. Through this hilus pass the ureter and the NAVL (nerve, artery, vein, and lymphatic) which service the kidney.

(3) Each kidney is attached to the posterior wall of the abdominal cavity, just above the waistline level. Each is held in place by special fascia and fat.

b. Gross Internal Structure. If we compare the structure of the kidney with that of a cantaloupe (muskmelon), the renal cortex would correspond to the hard rind, the renal medulla would correspond with the edible flesh of the melon, while the renal sinus would correspond to the hollow center (after the seeds have been removed). The medulla consists of pyramids with their bases at the cortex and forming peaks, papillae, which empty into the sinus.

PAPILLA = pimple, nipple

See figure 8-2 for a section of the kidney showing the inner structure.

c. The Nephron. See figure 8-3 for an illustration of a nephron. Nephrons are the functional units of the human kidney. Their primary function is to remove the wastes of protein usage from the blood. In addition, they serve to conserve water and other materials for continued use by the body. The end result of nephron function is a more or less concentrated fluid called urine. The kidneys contain great numbers of nephrons, about a million for each kidney. The main subdivisions of a nephron are the renal corpuscle and a tubular system.

(1) Renal corpuscle. The renal corpuscle has a hollow double- walled sac called the renal capsule ("Bowman's capsule"). Leading into the capsule is a very small artery called the afferent arteriole. Within the capsule, this artery becomes a mass of capillaries known as the glomerulus. An efferent arteriole drains the blood away from the capsule. The capsule and the glomerulus together are known as the renal corpuscle.

(2) Tubules. Each renal capsule is drained by a renal tubule. The first part of this tubule runs quite a distance in a coiled formation and is called the proximal convoluted tubule. A long loop, the renal loop (of Henle), extends down into the medulla with two straight parts and a sharp bend at the bottom. As the tube returns to the cortex layer, it once again becomes coiled and here is known as the distal convoluted tubule.

(3) Filtration/reabsorption. Except for the blood cells and the larger proteins, the fluid portion of the blood passes through the walls of the glomerulus into the cavity between the two layers of the renal capsule. This fluid is called the glomerular filtrate. By a process of taking back (resorption), the majority of the fluid is removed from the tubules and the concentrated fluid is called the urine.

d. The Collecting Tubule. The distal convoluted tubules of several nephrons empty into a collecting tubule. The urine is then passed from the collecting tubule at the papilla of the medullary pyramid. Several collecting tubules are present in each pyramid.

e. Renal Pelvis. The renal pelvis is a hollow sac within the sinus of the kidney. Urine from the pyramids collects into the funnel-shaped renal pelvis. The ureter then drains the urine from the renal pelvis.

URETERS

The ureters are tubes which connect the kidneys to the urinary bladder. The smooth muscle walls of the ureters produce a peristalsis (wave-like movement) that moves the urine along drop by drop.

URINARY BLADDER

a. The urinary bladder is a muscular organ for storing the urine. Near the inferior posterior corners of the urinary bladder are openings where the ureters empty into the bladder. Also at the inferior aspect of the urinary bladder is the exit, the beginning of the urethra. The triangular area, between the openings of the ureters and the urethra, is called the trigone, or base of the urinary bladder.

b. The urinary bladder wall is stretchable to accommodate varying volumes of urine.

c. Nerve endings called stretch receptors are found in the wall of the urinary bladder. Usually, the pressure within the urinary bladder is low. However, as the volume of the enclosed urine approaches the bladder's capacity, stretching of the wall stimulates the stretch receptors. The cycle of events controlling urination (voiding or emptying of the urinary bladder) is known as the voiding reflex.

URETHRA

The urethra is a tube which conducts the urine from the urinary bladder to the outside of the body. It begins at the anterior base of the urinary bladder.

a. Urethral Sphincters. The urethral sphincters are circular muscle masses which control the passage of the urine through the urethra. There are two urethral sphincters--an internal urethral sphincter and an external urethral sphincter.

(1) The internal urethral sphincter is located in the floor of the urinary bladder. It is made of smooth muscle tissue. It is controlled by nerves of the autonomic nervous system (lesson 11).

(2) The external urethral sphincter is more inferior around the urethra in the area of the pelvic floor. It is made up of striated muscle tissue. It is controlled by the peripheral nervous system (lesson 11).

b. Male-Female Differences. The female urethra is short and direct. The male urethra is much longer and has two curvatures. Whereas the female urethra serves only a urinary function, the male urethra serves both the urinary and reproductive functions.

Introduction to Human Genital (Reproductive) SystemSEXUAL DIMORPHISM

The human male and human female each has a system of organs specifically designed for the production of new humans. These systems are known as reproductive or genital systems. Since there are different systems for males and females, the genital systems are an example of sexual dimorphism.

MORPH = form, shape

DI = two

SEXUAL = according to sex (gender)

SEXUAL DIMORPHISM = having two different forms according to sex

ADVANTAGES OF DOUBLE PARENTING

The existence of two parents for each child means that genetic materials are recombined to produce a new type. This new type may be an improvement over previous generations.

MAJOR COMPONENT CATEGORIES OF THE GENITAL SYSTEMS

Components of the genital systems may be considered in the following categories:

a. Primary Sex Organs (Gonads). Primary sex organs produce sex cells (gametes). A male gamete and a female gamete may be united to form the one-cell beginning of an embryo (the process of fertilization). Primary sex organs also produce sex hormones.

b. Secondary Sex Organs. Secondary sex organs care for the product of the primary sex organ.

c. Secondary Sexual Characteristics. Secondary sexual characteristics are those traits that tend to make males and females more attractive to each other. Secondary sexual characteristics help to ensure mating. These characteristics first appear during puberty (10-15 years of age).

The Human Female Genital System

 PRIMARY SEX ORGANS (OVARIES)

The primary sex organ in the human female is the ovary. See figure 8-4 for an illustration of the female genital system. The ovaries are located to the sides of the upper end of the uterus. They are anchored to the posterior surface of the broad ligaments. (The broad ligaments are sheets or folds of peritoneum enclosing the uterus and uterine tubes and extending to the sides of the pelvis.)

a. The ovary produces the egg cell or ovum (ova, plural).

b. The ovary produces female sex hormones (estrogens and progesterone).

c. The production of ova is cyclic. One ovum is released in each menstrual period, about 28 days.

SECONDARY SEX ORGANS

a. Uterine Tubes (Fallopian Tubes, Oviducts). Extending to either side of the uterus are two muscular tubes which open at the outer ends like fringed trumpets. The fringe-like appendages encircle the ovaries. At their medial ends, the uterine tubes open into the uterus. The function of the uterine tubes is to pick up the ovum when released from the ovary and hold it UNTIL one of the following happens:

(1) It is fertilized. After fertilization, the initial stages of embryo development take place. The developing embryo is eventually moved into the uterus.

(2) The nutrient stored within the ovum is used up and the ovum dies. This may take three to five days.

b. Uterus. The uterus is the site where all but the first few days of embryo development takes place. After eight weeks of embryonic development, it is known as the fetus.

(1) Main subdivisions. The uterus is shaped like a pear, with the stem (cervix) facing downward and toward the rear. The fundus is the portion of the uterus above the openings of the uterine tubes. The main part, or body, is the portion between the cervix and the fundus. The uterus usually leans forward with the body slightly curved as it passes over the top of the urinary bladder. The cervix opens into the upper end of the vagina.

(2) Wall structure. The inner lining of the uterus is called the endometrium. Made up of epithelium, it is well supplied with blood vessels and glands. The muscular wall of the uterus is called the myometrium. In the body of the uterus, the muscular tissue is in a double spiral arrangement. In the cervix, it is in a circular arrangement.

(3) Age differences. The uterus of an infant female is undeveloped. During puberty, the uterus develops. The uterus of an adult is fully developed. The uterus of an old woman is reduced in size and nonfunctional.

c. Vagina. The vagina is a tubular canal connecting the cervix of the uterus with the outside. It serves as a birth canal and as an organ of copulation. It is capable of stretching during childbirth. The lower opening of the vagina may be partially closed by a thin membrane known as the hymen.

d. External Genitalia. Other terms for the external genitals of the human female are vulva and pudendum. Included are the:

(1) Mons pubis. The mons pubis is a mound of fat tissue covered with skin and hair in front of the symphysis pubis (the joint of the pubic bones).

(2) Labia majora. Extending back from the mons pubis and encircling the vestibule (discussed below) are two folds known as the labia majora. Their construction is similar to the mons pubis, including fatty tissue and skin. The outer surfaces are covered with hair. The inner surfaces are moist and smooth. The corresponding structure in the male is the scrotum.

LABIA = lips (LABIUM, singular)

(3) Labia minora. The labia minora are two folds of skin lying within the labia majora and also enclosing the vestibule. In front, each labium minus (minus = singular of minora) divides into two folds. The fold above the clitoris (discussed below) is called the prepuce of the clitoris. The fold below is the frenulum.

(4) Clitoris. The clitoris is a small projection of sensitive erectile tissue which corresponds to the male penis. However, the female urethra does not pass through the clitoris.

(5) Vestibule. The cleft between the labia minora and behind the clitoris is called the vestibule. It includes the urethral opening in front and the vaginal opening slightly to the rear.

e. Pregnancy and Delivery. When an embryo forms an attachment to the endometrium, a pregnancy exists. The attachment eventually forms a placenta, an organ joining mother and offspring for such purposes as nutrition of the offspring. The fetal membranes surround the developing individual (fetus) and are filled with amniotic fluid.

(1) During the first eight weeks, the developing organism is known as an embryo. During this time, the major systems and parts of the body develop.

(2) During the remainder of the pregnancy, the developing organism is known as the fetus. During this time, growth and refinement of the body parts occur.

(3) Parturition is the actual delivery of the fetus into a free- living state. The delivery of the fetus is followed by a second delivery-- that of the placenta and fetal membranes.

f. Menstruation and Menopause. About two weeks after an ovum is released, if it is not fertilized, menstruation occurs. Menstruation involves the loss of all but the basal layer of the endometrium. This process includes bleeding. It first occurs at puberty and lasts until menopause (45 to 55 years of age). After menopause, pregnancy is no longer possible.

SECONDARY SEXUAL CHARACTERISTICS

The secondary sexual characteristics of females include growth of pubic hair, development of mammary glands, development of the pelvic girdle, and deposition of fat in the mons pubis and labia majora.

MAMMARY GLANDS

 Secretion of milk begins after parturition. Stimulation from suckling helps to maintain the normal rate of milk secretion. At the time of menopause, breast tissue becomes less prominent.

The Human Male Genital System

 PRIMARY MALE SEX ORGANS (TESTES)

The primary sex organ of the human male is the testis. See figure 8-5 for an illustration of the male genital system. The testes are egg-shaped.

a. Location. The paired testes lie within the scrotum. The scrotum is a sac of loose skin attached in the pubic area of the lower abdomen. The scrotum provides a site cooler than body temperature to maintain the viability of the spermatozoa. However, when the air is too cold, muscles and muscular fibers draw the testes and scrotum closer to the body to maintain warmth. Otherwise, the scrotum hangs loosely. The tunica vaginalis is a serous cavity surrounding each testis.

b. Functions. The testis produces the male sex cells called spermatozoa (spermatozoon, singular). The spermatozoa are continuously produced by the millions. One such cell may eventually fertilize an ovum of a human female. The testes also produce male sex hormones called androgens.

SECONDARY SEX ORGANS

a. Epididymis. The epididymis is a coiled tube whose function is to aid in the maturation of spermatozoa. Its coiled length is only about one and one-half inches. Its uncoiled length is about 16 feet. When coiled, it extends downward along the posterior side of each testis. Its lining secretes a nutritive medium for spermatozoa. It receives spermatozoa from the testes in an immature state. As the spermatozoa pass through the nutrient, they mature.

b. Ductus (Vas) Deferens. The ductus deferens is a transporting tube which carries the mature sperm from the epididymis to the prostate. Each tube enters the abdomen through the inguinal canal. Each passes over a ureter to reach the back of the urinary bladder and then down to the prostate gland.

c. Seminal Vesicles. Lying alongside each ductus deferens as it crosses the back of the bladder is a tubular structure called the seminal vesicle. The seminal vesicle produces a fluid which becomes part of the ejaculate.

d. Ejaculatory Duct. Each ductus deferens and its corresponding seminal vesicle converge to form a short tube called the ejaculatory duct. The ejaculatory duct opens into the urethra within the prostate gland. The ejaculatory duct carries both spermatozoa and seminal vesicle fluid.

e. Prostate Gland. As the urethra leaves the urinary bladder, its first inch is surrounded by a chestnut-size gland called the prostate gland. The prostate gland provides an additional fluid to be added to the spermatozoa and seminal vesicle fluid.

f. Penis. As the urethra leaves the abdomen, it passes through the penis, the male organ of copulation.

(1) Surrounding the urethra is a central cylinder of erectile tissue called the corpus spongiosum. This cylinder is bulb-shaped at each end. The posterior end is attached to the base of the pelvis. The sensitive anterior end is known as the glans.

CORPUS SPONGIOSUM = spongy body

(2) Overlying the corpus spongiosum is a pair of cylinders of erectile tissue called the corpora cavernosa. These two cylinders are separate in their proximal fourth and joined in their distal three-fourths. They are attached to the pubic bones. Together, the corpus spongiosum and the corpora cavernosa combine to form the shaft of the penis.

CORPUS CAVERNOSUM = cavernous body

(3) The prepuce, or foreskin, is a covering of skin for the glans. It may be removed in a surgical procedure called circumcision.

SECONDARY SEXUAL CHARACTERISTICS

The secondary sexual characteristics of male include growth of facial, pubic, and chest hair; growth of the larynx to deepen the voice; and deposition of protein to increase muscularity and general body size.

The Respiratory System

INTRODUCTION

a. Respiration. Respiration is the exchange of gases between the atmosphere and the cells of the body. It is a physiological process. There are two types of respiration--external and internal. External respiration is the exchange of gases between the air in the lungs and blood. Internal respiration is the exchange of gases between the blood and the individual cells of the body.

b. Breathing. Breathing is the process that moves air into and out of the lungs. It is a mechanical process. There are two types of breathing in humans--costal (thoracic) and diaphragmatic (abdominal). In costal breathing, the major structure causing the movement of the air is the rib cage. In diaphragmatic breathing, interaction between the diaphragm and the abdominal wall causes the air to move into and out of the lungs.

COMPONENTS AND SUBDIVISIONS OF THE HUMAN RESPIRATORY SYSTEM

a. Components. The components of the human respiratory system consist of air passageways and two lungs. Air moves from the outside of the body into tiny sacs in the lungs called alveoli (pronounced al-VE-oh-lie).

b. Main Subdivisions. The main subdivisions of the respiratory system may be identified by their relationship to the voice box or larynx. Thus, the main subdivisions are as listed in table

SUBDIVISION	FUNCTION
(1) SUPRALARYNGEAL STRUCTURES (su-prah-lah-RIN-je-al)	Cleanse, warm, moisten, and test inflowing air
(2) LARYNX (voice box) (LARE-inks)	Controls the volume of inflowing air; produces selected pitch(vibration frequency) in the moving column of air
(3) INFRALARYNGEAL STRUCTURES (in-frah-lah-RIN-je-al)	Distribute air to the alveoli of the lung where the actual external respiration takes place

SUPRALARYNGEAL STRUCTURES

See figure 7-2.

a. External Nose. The external nose is the portion projecting from the face. It is supported primarily by cartilages. It has a midline divider called the nasal septum, which extends from the internal nose. Paired openings (nostrils) lead to paired spaces (vestibules). Guard hairs in the nostrils filter inflowing air.

b. Nasal Chambers (Internal Nose). Behind each vestibule of the external nose is a nasal chamber. The two nasal chambers together form the internal nose. These chambers too are separated by the nasal septum.

(1) Mucoperiosteum. The walls of the nasal chambers are lined with a thick mucous-type membrane known as the mucoperiosteum. It has a ciliated epithelial surface and a rich blood supply, which provides warmth and moisture. At times, it may become quite swollen.

CILIATED = provided with cilia (hairlike projections which move fluids to the rear)

(2) Conchae. The lateral wall of each chamber has three scroll- like extensions into the nasal chamber which help to increase the surface area exposed to the inflowing air. These scroll-like extensions are known as conchae.

CONCHA (pronounced KON-kah) = sea shell

CONCHA (singular), CONCHAE (plural)

(3) Olfactory epithelium. The sense of smell is due to special nerve endings located in the upper areas of the nasal chambers. The epithelium containing the sensory endings is known as the olfactory epithelium.

(4) Paranasal sinuses. There are air "cells" or cavities in the skull known as paranasal sinuses. The paranasal sinuses are connected with the nasal chambers and are lined with the same ciliated mucoperiosteum. Thus, these sinuses are extensions of the nasal chambers into the skull bones. For this reason, they are known as paranasal sinuses.

c. Pharynx. The pharynx (FAIR-inks) is the common posterior space for the respiratory and digestive systems.

(1) Nasopharynx. That portion of the pharynx specifically related to the respiratory system is the nasopharynx. It is the portion of the pharynx above the soft palate. The two posterior openings (nares) of the nasal chambers lead into the single space of the nasopharynx. The auditory (eustachian) tubes also open into the nasopharynx. The auditory tubes connect the nasopharynx with the middle ears (to equalize the pressure between the outside and inside of the eardrum). Lying in the upper posterior wall of the nasopharynx are the pharyngeal tonsils (adenoids). The soft palate floor of the nasopharynx is a trapdoor which closes off the upper respiratory passageways during swallowing.

(2) Oropharynx. The portion of the pharynx closely related to the digestive system is the oropharynx. It is the portion of the pharynx below the soft palate and above the upper edge of the epiglottis. (The epiglottis is the flap that prevents food from entering the larynx (discussed below) during swallowing.)

(3) Laryngopharynx. That portion of the pharynx which is common to the respiratory and digestive systems is the laryngopharynx. It is the portion of the pharynx below the upper edge of the epiglottis. Thus, the digestive and respiratory systems lead into it from above and lead off from it below.

LARYNX

The larynx, also called the Adam's apple or voice box, connects the pharynx with the trachea. The larynx, located in the anterior neck region, has a box-like shape. See figure 7-3 for an illustration. Since the voice box of the male becomes larger and heavier during puberty, the voice deepens. The adult male's voice box tends to be located lower in the neck; in the female, the larynx remains higher and smaller and the voice is of a higher pitch.

a. Parts and Spaces. The larynx has a vestibule ("entrance hallway") which can be covered over by the epiglottis. The glottis itself is the hole between the vocal cords. Through the glottis, air passes from the vestibule into the main chamber of the larynx (below the cords) and then into the trachea. The skeleton of the larynx is made up of a series of cartilages.

b. Muscles. The larynx serves two functions and there are two sets of muscles--one for each function.

(1) One set controls the size of the glottis. Thus, it regulates the volume of air passing through the trachea.

(2) The other set controls the tension of the vocal cords. Thus, it produces vibrations of selected frequencies (variations in pitch) of the moving air to be used in the process of speaking.

INFRALARYNGEAL STRUCTURES

a. Trachea and Bronchi. The respiratory tree (figure 7-4) is the set of tubular structures which carry the air from the larynx to the alveoli of the lungs. Looking at a person UPSIDE DOWN, the trachea is the trunk of the tree and the bronchi are the branches. These tubular parts are held open (made patent) by rings of cartilage. Their lining is ciliated to remove mucus and other materials that get into the passageway.

b. Alveoli. The alveoli (alveolus, singular) are tiny spherical (balloon-like) sacs which are connected to the larger tubes of the lungs by tiny tubes known as alveolar ducts and bronchioles. The alveoli are so small that there are billions in the adult lungs. This very small size produces a maximum surface area through which external respiration takes place. External respiration is the actual exchange of gases between the air in the alveolar spaces and the adjacent blood capillaries through their walls.

c. Lungs. A lung is an individual organ composed of tubular structures and alveoli bound together by fibrous connective tissue (FCT). In the human, there are two lungs--right and left. Each lung is supplied by a primary or mainstem bronchus leading off of the trachea. The right lung is larger in volume than the left lung. The left lung must leave room for the heart. The right lung is divided into three pulmonary lobes (upper, middle, and lower) and 10 bronchopulmonary segments (2 + 3 + 5). The left lung is divided into two pulmonary lobes (upper and lower) and eight bronchopulmonary segments (4 + 4). A pulmonary lobe is a major subdivision of a lung marked by fissures (deep folds). Each lobe is further partitioned into bronchopulmonary segments. Each lobe is supplied by a secondary or lobar bronchus. Each segment is supplied by a tertiary or segmental bronchus, a branch of the lobar bronchus.

d. Pleural Cavities. See paragraph 3-14 to review a description of pleural cavities. That paragraph indicates that each serous cavity has inner and outer membranes. In the case of the lungs, the inner membrane is known as the visceral pleura which very closely covers the surface of the lungs. The outer membrane is known as the parietal pleura, forming the outer wall of the cavity. The pleural cavities are the potential spaces between the inner and outer membranes. The pleural cavities allow the lungs to move freely with a minimum of friction during the expansion and contraction of breathing.

Breathing and Breathing Mechanisms
   INTRODUCTION

a. Boyle's law tells us that as the volume (V) of a gas-filled container increases, the pressure (P) inside decreases; as the volume (V) of a closed container decreases, the pressure (P) inside increases. When two connected spaces of air have different pressures, the air moves from the space with greater pressure to the one with lesser pressure. In regard to breathing, we can consider the air pressure around the human body to be constant. The pressure inside the lungs may be greater or less than the pressure outside the body. Thus, a greater internal pressure causes air to flow out; a greater external pressure causes air to flow in.

b. We can compare the human trunk to a hollow cylinder. This cylinder is divided into upper and lower cavities by the diaphragm. The upper is the thoracic cavity and is essentially gas-filled. The lower is the abdominopelvic cavity and is essentially water-filled.

  COSTAL (THORACIC) BREATHING

a. Inhalation. Muscles attached to the thoracic cage raise the rib cage. A typical rib might be compared to a bucket handle, attached at one end to the sternum (breastbone) and at the other end to the vertebral column. The "bucket handle" is lifted by the overall movement upward and outward of the rib cage. These movements increase the thoracic diameters from right to left (transverse) and from front to back (A-P). Thus, the intrathoracic volume increases. Recalling Boyle's law, the increase in volume leads to a decrease in pressure. The air pressure outside the body then forces air into the lungs and inflates them.

b. Exhalation. The rib cage movements and pressure relationships are reversed for exhalation. Thus, intrathoracic volume decreases. The intrathoracic pressure increases and forces air outside the body.

DIAPHRAGMATIC (ABDOMINAL) BREATHING

The diaphragm is a thin, but strong, dome-shaped muscular membrane that separates the abdominal and thoracic cavities. The abdominal wall is elastic in nature. The abdominal cavity is filled with soft, watery tissues.

a. Inhalation. As the diaphragm contracts, the dome flattens and the diaphragm descends. This increases the depth (vertical diameter) of the thoracic cavity and thus increases its volume. This decreases air pressure within the thoracic cavity. The greater air pressure outside the body then forces air into the lungs.

b. Exhalation. As the diaphragm relaxes, the elastic abdominal wall forces the diaphragm back up by pushing the watery tissues of the abdomen against the underside of the relaxed diaphragm. The dome extends upward. The process of inhalation is thus reversed.

       

The Human Digestive System

Lesson 1 Introduction

GENERAL

a. Definition. The human digestive system is a group of organs designed to take in foods, initially process foods, digest the foods, and eliminate unused materials of food items. It is a hollow tubular system from one end of the body to the other end.

See figure 6-1.

b. Major Organs. The major organs involved in the human digestive system are listed below. They are each discussed later in this lesson.

(1) Mouth or oral complex.

(2) Pharynx.

(3) Esophagus.

(4) Stomach.

(5) Small intestines and associated glands.

(6) Large intestines.

(7) Rectum.

(8) Anal canal and anus.

c. Digestive Enzymes. A catalyst is a substance that accelerates (speeds up) a chemical reaction without being permanently changed or consumed itself. A digestive enzyme serves as a catalyst, aiding in digestion. Digestion is a chemical process by which food is converted into simpler substances that can be absorbed or assimilated by the body. Enzymes are manufactured in the salivary glands of the mouth, in the lining of the stomach, in the pancreas, and in the walls of the small intestine.

FOODS AND FOODSTUFFS

Examples of food items are a piece of bread, a pork chop, and a tomato. Food items contain varying proportions of foodstuffs. Foodstuffs are the classes of chemical compounds which make up food items. The three major types of foodstuffs are carbohydrates, lipids (fats and oils), and proteins. Food items also contain water, minerals, and vitamins.

ORAL COMPLEX

The oral complex consists of the structures commonly known together as the mouth. It takes in and initially processes food items. See figure 6-2.

a. Teeth.

(1) A tooth (figure 6-3) has two main parts--the crown and the root. A root canal passes up through the central part of the tooth. The root is suspended within a socket (called the alveolus) of one of the jaws of the mouth. The crown extends up above the surface of the jaw. The root and inner part of the crown are made of a substance called dentin. The outer portion of the crown is covered with a substance known as enamel. Enamel is the hardest substance of the human body. The nerves and blood vessels of the tooth pass up into the root canal from the jaw substance.

(2) There are two kinds of teeth-- anterior and posterior. The anterior teeth are also known as incisors and canine teeth. The anterior teeth serve as choppers. They chop off mouth-size bites of food items. The posterior teeth are called molars. They are grinders. They increase the surface area of food materials by breaking them into smaller and smaller particles.

(3) Humans have two sets of teeth--deciduous and permanent. Initially, the deciduous set includes 20 baby teeth.

DECIDUOUS = to be shed

These are eventually replaced by a permanent set of 32.

b. Jaws. There are two jaws--the upper and the lower. The upper is called the maxilla. The lower is called the mandible.

(1) In each jaw, there are sockets for the teeth. These sockets are known as alveoli. The bony parts of the jaws holding the teeth are known as alveolar ridges.

(2) The upper jaw is fixed to the base of the cranium. The lower jaw is movable. There is a special articulation (T-MJ--temporo-mandibular joint) with muscles to bring the upper and lower teeth together to perform their functions.

c. Palate. The palate serves as the roof of the mouth and the floor of the nasal chamber above. Since the anterior two-thirds is bony, it is called the hard palate. The posterior one-third is musculo-membranous and is called the soft palate. The soft palate serves as a trap door to close off the upper respiratory passageway during swallowing.

d. Lips and Cheeks. The oral cavity is closed by a fleshy structure around the opening. Forming the opening are the lips. On the sides are the cheeks.

e. Tongue. The tongue is a muscular organ. The tongue is capable of internal movement to shape its body. It is moved as a whole by muscles outside the tongue. Interaction between the tongue and cheeks keeps the food between the molar teeth during the chewing process. When the food is properly processed, the tongue also initiates the swallowing process.

f. Salivary Glands. Digestion is a chemical process which takes place at the wet surfaces of food materials. The chewing process has greatly increased the surface area available. The surfaces are wetted by saliva produced by glands in the oral cavity. Of these glands, three pairs are known as the salivary glands proper.

g. Taste Buds. Associated with the tongue and the back of the mouth are special clumps of cells known as taste buds. These taste buds literally taste the food. That is, they check its quality and acceptability.

PHARYNX

The pharynx (pronounced "FAIR -inks") is a continuation of the rear of the mouth region, just anterior to the vertebral column (spine). It is a common passageway for both the respiratory and digestive systems.

ESOPHAGUS

The esophagus is a muscular, tubular structure extending from the pharynx, down through the neck and the thorax (chest), and to the stomach. During swallowing, the esophagus serves as a passageway for the food from the pharynx to the stomach.

STORAGE FUNCTION

The stomach is a sac-like enlargement of the digestive tract specialized for the storage of food. Since food is stored, a person does not have to eat continuously all day. One is freed to do other things. The presence of valves at each end prevents the stored food from leaving the stomach before it is ready. The pyloric valve prevents the food from going further. The inner lining of the stomach is in folds to allow expansion.

DIGESTIVE FUNCTION

a. While the food is in the stomach, the digestive processes are initiated by juices from the wall of the stomach. The musculature of the walls thoroughly mixes the food and juices while the food is being held in the stomach. In fact, the stomach has an extra layer of muscle fibers for this purpose.

b. When the pyloric valve of the stomach opens, a portion of the stomach contents moves into the small intestine.

GENERAL

a. Digestion is a chemical process. This process is facilitated by special chemicals called digestive enzymes. The end products of digestion are absorbed through the wall of the gut into the blood vessels. These end products are then distributed to body parts that need them for growth, repair, or energy.

b. There are associated glands--the liver and the pancreas--which produce additional enzymes to further the process.

c. Most digestion and absorption takes place in the small intestines.

ANATOMY OF THE SMALL INTESTINES

a. The small intestines are classically divided into three areas-- the duodenum, the jejunum, and the ileum. The duodenum is C-shaped, about 10 inches long in the adult. The duodenum is looped around the pancreas.

DUODENUM = 12 fingers (length equal to width of 12 fingers)

The jejunum is approximately eight feet long and connects the duodenum and ileum. The ileum is about 12 feet long. The jejunum and ileum are attached to the rear wall of the abdomen with a membrane called a mesentery. This membrane allows mobility and serves as a passageway for nerves and vessels (NAVL) to the small intestines.

JEJUNUM = empty

ILEUM = lying next to the ilium (bone of the pelvic girdle; PELVIS = basin)

b. The small intestine is tubular. It has muscular walls which produce a wave-like motion called peristalsis moving the contents along. The small intestine is just the right length to allow the processes of digestion and absorption to take place completely.

c. The inner surface of the small intestine is NOT smooth like the inside of new plumbing pipes. Rather, the inner surface has folds (plicae). On the surface of these plicae are finger-like projections called villi (villus, singular). This folding and the presence of villi increase the surface area available for absorption.

LIVER AND GALLBLADDER

a. Liver Anatomy. The liver is a large and complex organ. Most of its mass is on the right side of the body and within the lower portion of the rib cage. Its upper surface is in contact with the diaphragm.

b. Liver Functions. The liver is a complex chemical factory with many functions. These include aspects of carbohydrate, protein, lipid, and vitamin metabolism and processes related to blood clotting and red blood cell destruction. Its digestive function is to produce a fluid called bile or gall.

c. Gallbladder. Until needed, the bile is stored and concentrated in the gallbladder, a sac on the inferior surface of the liver. Fluid from the gallbladder flows through the cystic duct, which joins the common hepatic duct from the liver to form the common bile duct. The common bile duct then usually joins with the duct of the pancreas as the fluid enters the duodenum.

PANCREAS

The pancreas is a soft, pliable organ stretched across the posterior wall of the abdomen. When called upon, it secretes its powerful digestive fluid, known as pancreatic juice, into the duodenum. Its duct joins the common bile duct.

Large Intestine

The primary function of the large intestines is the salvaging of water and electrolytes (salts). Most of the end products of digestion have already been absorbed in the small intestines. Within the large intestines, the contents are first a watery fluid. Thus, the large intestines are important in the conservation of water for use by the body. The large intestines remove water until a nearly solid mass is formed before defecation, the evacuation of feces.

MAJOR SUBDIVISIONS

The major subdivisions of the large intestines are the cecum (with vermiform or "worm-shaped" appendix), the ascending colon, the transverse colon, the descending colon, and the sigmoid colon. The fecal mass is stored in the sigmoid colon until passed into the rectum.

RECTUM, ANAL CANAL, AND ANUS

Rectum means "straight." However, this six-inch tubular structure would actually look a bit wave-like from the front. From the side, one would see that it was curved to conform the sacrum (at the lower end of the spinal column). The final storage of feces is in the rectum. The rectum terminates in the narrow anal canal, which is about one and one-half inches long in the adult. At the end of the anal canal is the opening called the anus. Muscles called the anal sphincters aid in the retention of feces until defecation.

Associate Protective Structures
 Within the body, there are many structures that aid in protection from bacteria, viruses, and other foreign substances. These structures include cells that can phagocytize (engulf) foreign particles or manufacture antibodies (which help to inactivate foreign substances). Collectively, such cells make up the reticuloendothelial system (RES). Such cells are found in bone marrow, the spleen, the liver, and lymph nodes.

STRUCTURES WITHIN THE DIGESTIVE SYSTEM

Lymphoid structures make up the largest part of the RES. Lymphoid structures are collections of cells associated with circulatory systems (to be discussed in lesson 9).

a. Tonsils are associated with the posterior portions of the respiratory and digestive areas in the head, primarily in the region of the pharynx. The tonsils are masses of lymphoid tissue.

b. Other lymphoid aggregations are found in the walls of the small intestines.

c. The vermiform appendix, attached to the cecum of the large intestine, is also a mass of lymphoid tissue. It is the "tonsil" of the intestines.

The Human Muscular System

The Skeletal Muscle

MUSCLE TISSUES

a. Smooth muscle tissue is utilized to make up the muscular portion of the various visceral organs (stomach, blood vessels, etc.).

b. Cardiac muscle tissue makes up the muscular wall of the heart--the myocardium.

c. Striated muscle tissue is used in the makeup of several types of muscles. The main type of muscle is the skeletal muscle. Other types of muscles made with striated muscle tissue are the facial or integumentary muscles and muscles of the jaw apparatus.


THE SKELETAL MUSCLE

Each skeletal muscle is an individual organ of the human body. Each is made up of several types of tissues--mainly, striated muscle fibers and FCT (fibrous connective tissue). Each is attached to and moves bones. Bones are parts of the skeleton serving as levers.

a. General Construction of a Skeletal Muscle. The large portion of a muscle is known as its belly or fleshy belly. This muscle is attached to bones by tendons or aponeuroses. Tendons and aponeuroses are similar to each other. However, tendons are cord-like and aponeuroses are broad and flat. The fleshy portion may be directly connected to the bone. If so, it is called a "fleshy attachment."

b. Muscular NAVL (Nerves, Arteries, Veins, Lymphatics).

(1) From the main NAVL (nerve, artery, vein, lymphatic), there are branches going to each muscle. These muscular branches are bound together by an FCT sheath to form a neurovascular bundle.

(2) The motor point is that specific location on the surface of the muscle where the neurovascular bundle enters.

(3) A motor unit is the single motor neuron and the number of striated muscle fibers activated by it (innervation). The importance of the motor unit is that its fibers work in unison. Either all fibers within a unit contract or none contract. When a certain amount of force is needed, one unit after another is recruited until just enough units are available to produce the desired action.

NAMING SKELETAL MUSCLES

The name of a muscle may appear with the abbreviation M., meaning Musculus or muscle. We abbreviate muscles (plural) with the symbol Mm. Skeletal muscles are named according to their physical attributes (shape, size, length, etc.), their location, or their function. For example:

SHAPE:	deltoid M. DELTA = D , Greek letter Dbiceps M. BICEPS = two-head BI = two CEPS = head
SIZE:	adductor magnus M. MAGNUS = great, large
LENGTH:	adductor longus M. LONGUS = long
LOCATION:	biceps brachii M. BRACHII = of the armbiceps femoris M. FEMORIS = of the thigh
FUNCTION:	rotatores Mm. ROTATORES = rotators (They turn/rotate the vertebral column.)

ARRANGEMENT OF HUMAN SKELETAL MUSCLES
          

a. Trunk Musculature. The trunk musculature is arranged in two ways--longitudinal muscles and oblique muscles. Together, they:

(1) Maintain trunk posture.

(2) Move the parts of the trunk.

(3) Adjust the internal pressures of the trunk to perform certain functions such as breathing.

b. Limb Musculature. The limb musculature is arranged around the joints to produce the appropriate motions of the limbs. Elementary mechanics are described in the next section to help you to understand typical arrangements of limb musculature.

Some Elementary Skeleto-Muscular Mechanics
GENERAL

Muscles and bones together work like machines within the laws of physics and chemistry. Lever and pulley systems are examples of simple machines found commonly in the human body.

SIMPLE PULLEY SYSTEM

a. In the human body when the tendon of a skeletal muscle slides over a round bony surface, the "system" acts like a simple pulley (figure 5-4). A simple pulley provides a change in the direction of the force or muscle pull. There is no change in the amount of force produced by the muscle. For example, the knee acts as a simple pulley by which the quadriceps femoris M. extends the leg.

b. Sesamoid bones, such as the patella (kneecap), develop in tendons where pressure is applied to the tendon.

The Human Skeletal System

INTRODUCTION

The skeleton serves as a support or framework of the human body. It is a combination of bones joined together.

FUNCTIONS OF THE HUMAN SKELETON

The human skeleton serves the following functions:

a. Bodily Support. The skeletal system provides a framework for the human body.

b. Protection. The skeleton protects certain soft structures within the human body. An example is the skull, which surrounds the brain.

c. Motion. Muscles are attached to and move the bones. Bones provide leverage for motion.

d. Formation of Blood Cells (Hematopoiesis). Blood cells are manufactured in the red bone marrow, mainly found in flat bones.

PRIMARY STUDY AREAS

In this text, we study the skeletal system from four different viewpoints:

a. Bone As Tissues.

b. Bone As An Individual Organ.

c. Articulations (Joints)--Arthrology.

d. The Human Skeleton.

BASIC STRUCTURE OF AN INDIVIDUAL BONE

a. Use of Bony Tissues to Form an Individual Bone.

(1) Cortex. The cortex is the outer layer of the individual bone. It is made up of compact (dense) bony tissue.

(2) Medulla. The medulla is the central portion of the individual bone. It generally consists of cancellous (spongy) bone tissue. In some bones, particularly long bones, the medulla may include a space without any bony tissue. This space is called the medullary or marrow cavity.

b. Marrow. Marrow serves as a filler of the inside of bones. There are two types of bone marrow--yellow bone marrow and red bone marrow. Yellow bone marrow is mostly yellow fat tissue. Red bone marrow is the only site in adults for the formation of red blood cells (hematopoiesis).

c. Named Parts of an Individual Long Bone.

(1) Shaft (diaphysis). The shaft is the central portion of a long bone. Here, the cortex is thickened as required by applied physical stresses.

(2) Ends (epiphyses). The ends of long bones are made up mainly of cancellous (spongy) bone tissue. An articular cartilage covers each area where a bone contacts another bone(s). This articular cartilage is made up of hyaline cartilage tissue and provides a smooth surface for motions.

d. Periosteum. The periosteum is a covering of the bone surface area not covered by articular cartilage. It has two layers--the innermost layer and the fibrous layer.

(1) The innermost layer, which lies against the outer surface of the bone, consists of bone-forming cells (osteoblasts). It is the osteogenic (bone-forming) layer.

(2) The outermost layer is a FCT (fibrous connective tissue) layer.

(3) The periosteum is well supplied with blood vessels and sensory-type nervous tissue.

e. Blood Supply of an Individual Bone. A system of blood vessels enters and spreads out through the periosteum. Additional blood vessels, called "nutrient vessels," penetrate the cortex of the bone and spread out through the marrow. The passageways for penetration of these vessels are called the nutrient canals.    

DEVELOPMENT OF AN INDIVIDUAL BONE

a. General. The human skeleton is "preformed" in the early fetus, but the early form is not of bony material. There are two types of bones according to their preformed basis: membranous bones and cartilage bones. These are in the location and have the general shape of the adult bones they will later become.

(1) Membranous bones. The outer skull bones are an example of membranous bones. Osteoblasts invade a membrane to form a center of ossification (formation of bone). Bone-forming activity spreads out from this center until a full bone plate is formed.

(2) Cartilage bones. In the fetus, many bones, for example, long bones, exist first as models formed of cartilage.

b. Sesamoid Bones. Sesamoid bones are small masses of bone that develop in tendons at points where great forces are applied to the tendons. The most obvious and largest sesamoid bone is the patella, or kneecap.

c. Ossification Centers. An ossification center is a growing mass of actual bone within the preformed material, as noted above.

(1) Initial bone formation involves destruction of the preforming material and replacement with bony tissue.

(2) In the development of long bones, there are two types of ossification centers:

(a) Diaphyseal--in the shaft region.

(b) Epiphyseal--in the end(s).

(3) As a long bone grows in length, the preforming material grows faster than the ossification center can tear it down. Ultimately, with time, the preforming material is overcome and growth ceases.

d. Growth in Bone Width. A bone grows wider through the activity of the osteogenic layer of the periosteum. Remember, the periosteum covers most of the outer surface of the bone.

TYPES OF BONES

Bones of the skeleton can be grouped into the following major types: long, short, flat, and irregular. Each type has a somewhat different construction pattern.

a. Long Bones. The basic structure of a long bone is illustrated in figure 4-1 and discussed in module  4-4. Example: femur.

b. Short Bones. The short bones, such as those of the wrist and feet, have a thin layer of compact bone surrounding an inner mass of spongy bone.

Example: carpal bones.

c. Flat Bones. The flat bones are constructed with two plates of compact bone, which enclose between them a layer of spongy bone. The spongy bone is richly supplied with blood vessels and red marrow. Example: the cranial frontal bone.

d. Irregular Bones. The irregular bones are those that do not fit into the three categories above. Example: a vertebra.

Arthrology--The Study of Joints (Articulations)
DEFINITION

A joint, or articulation, is the location where two or more bones meet.

TYPES OF JOINTS

Joints are classified according to the kind of material holding the bones together and the relative freedom and kind of motion at the particular joint.

a. Fibrous Joints. Varying degrees of motion, from none to some, are possible in fibrous joints.

(1) Syndesmosis. When the bones are held together by FCT (fibrous connective tissue), the joint is referred to as a syndesmosis.

SYN = together

DESMOS = fiber (a tying material)

Example: The inferior tibio-fibular joint.

(2) Suture. When the bones are quite close together with a minimum of FCT, the joint is known as a suture. Example: the joints between the cranial bones.

b. Bony Joints. Should the bones be united by bony material, the joint is referred to as a synosteosis.

SYN = together

OSTEO = bone

Example: The frontal bone. (The frontal bone of the skull is actually a bony fusion of two bones. Approximately 10 percent of the time, this fusion fails to take place; the original suture between the bones remains and is called a metopic suture.)

c. Cartilagenous Joints. These are also nonmovable joints.

(1) Synchondrosis. A cartilagenous joint in which the bones are held together by hyaline cartilage.

SYN = together

CHONDRO = cartilage

Example: Epiphyseal plate.

(2) Symphysis. A cartilagenous joint in which the bones are held together by a disc of fibrocartilage.

Example: Pubic symphysis.

d. Synovial Joints. In the synovial type of joints, the bones move on one another so as to allow various motions of the body parts. The "ovial" part of the name refers to the fact that the fluid substance seen in this type of joint appeared to the old anatomists to be like raw egg white (ovum = egg).

"TYPICAL" SYNOVIAL JOINT

A "typical" synovial joint is one which has parts common to all of the synovial joints. In a sense, it is imaginary. It is not actually a specific synovial joint. It is a composite. It is illustrated in figure 4-2. The "typical" synovial joint has the following parts:

a. Bones. Bones are the levers of motion. They are the site of attachment for skeletal muscles.

b. Articular Cartilages. The "contact" points of the bones are usually covered with a layer of lubricated cartilage. Where these cartilages end, the synovial membranes begin. Cartilages provide a smooth surface to reduce friction.    

  

c. Synovial Membrane, Space, and Fluid.

(1) Synovial membrane. The synovial membrane lines the inner surface of the capsule. It secretes synovial fluid into the synovial space.

(2) Synovial space. Figure 4-2 exaggerates the amount of space between the bones. The space within the capsule allows movement.

(3) Synovial fluid. Synovial fluid is a colorless, viscous fluid similar in consistency to raw egg white. It lubricates the articulation.

d. Capsule. The "typical" synovial articulation is surrounded by a sleeve of dense FCT known as the capsule. The capsule encloses the articulation.

e. Ligaments. Primarily, ligaments hold bones together. Ligaments also may help restrain motion in certain directions and stabilize the articulation.

f. Muscles. Skeletal muscles apply the forces to produce a given motion.

CLASSIFICATION OF SYNOVIAL JOINTS

Synovial joints are further classified according to the kind of motion and the number of axes of motions used.

a. Uni-Axial Synovial Joints.

(1) In uni-axial synovial joints, motion occurs in only one plane. The joints of the fingers (interphalangeal) flex and extend in the sagittal plane. These are commonly referred to as hinge joints.

(2) If a single rotatory (rotational) motion occurs around a post-like structure, the joint is a pivot joint. The atlas vertebra rotating around the dens (tooth like projection) of the axis vertebra at the top of the neck (base of the skull) is a pivot joint.

b. Bi-Axial Synovial Joints. In bi-axial synovial joints, motion between the bones occurs in two planes. Here the surface in contact is curved or rounded in two directions.

(1) The proximal phalanx of a finger can flex and extend and move from side to side on the rounded head of the metacarpal bone. This is the MP or metacarpophalangeal joint.

(2) When the two surfaces are curved in directions at right angles to each other, a shape similar to that of a cowboy's saddle is formed. This type of synovial joint is called a saddle joint. In the human body, the saddle joint is located at the base of the thumb.

c. Multi-Axial Synovial Joints. In multi-axial joints, motion is possible in all three planes of space.

(1) The ball-and-socket-type synovial joint has the freest motion in all directions. A spherically rounded head (ball-like) fits into a receiving concavity (socket). The hip joint is an example of the ball-and-socket type, with the spherical head of the femur fitting into the cup or socket (acetabulum) of the pelvic bone.

(2) In the plane joint, the contact surfaces of the bones are essentially flat. These flat surfaces slide on one another (also called translatory motion). The acromioclavicular joint of the shoulder region is an example of a plane joint.

THE ARTICULAR DISC

In three of the synovial joints of the human body, a special addition is seen. This addition is known as an articular disc. The joints with articular discs are the temporomandibular joint of the lower jaw, the sternoclavicular joint (at the sternum (breastbone)), and the ulnocarpal joint of the distal end of the forearm.

a. An articular disc is a fibrocartilage plate. It is inserted between the articular surfaces of the bones of a synovial joint. In this way, it divides the synovial space into two spaces.

b. Joints having an articular disc are capable of having several different motions occurring at the same time. Mechanically, there are really two joints together here.

The Human Skeleton
GENERAL

a. The human skeleton is a collection of individual bones articulated (joined) together.

b. The major subdivisions of the skeleton are the axial skeleton and the appendicular skeleton.

THE AXIAL SKELETON

The axial skeleton is the central framework of the human body. It includes the skull, the vertebral column (spine), and the thoracic cage (chest or rib cage).

a. Vertebral Column (Spine). The vertebral column, or spine, is made up of a vertical series of bony blocks called vertebrae. These vertebrae are joined together in such a way as to form a semiflexible rod. The spine is the central support for the trunk, yet allows trunk movements.

(1) Anatomically and functionally, a typical vertebra (figure 4-4) is constructed of two major parts:

(a) The vertebral body is a drum-shaped cylindrical mass. Its superior and inferior surfaces are flat. Its function is primarily weight-bearing.

(b) The neural arch extends posteriorly, arching over and protecting the spinal cord of the central nervous system. From the neural arch are several processes. These processes serve as attachment areas for the trunk muscles. They also act as levers during various trunk motions.

(2) The vertebral column has 32-33 vertebrae, one on top of the other. These vertebrae are arranged in regions. The vertebrae of each region have a characteristic shape. The regions are as follows:

(a) Cervical (neck) region, with seven cervical vertebrae.

(b) Thoracic (chest) region, with 12 thoracic vertebrae.

(c) Lumbar (low back) region with five lumbar vertebrae.

(d) The sacrum, which is a bony fusion of five sacral vertebrae.

(e) The coccyx (pronounced COCK-sicks, "tail"), with 3-4 coccygeal vertebrae together.

(3) The vertebrae are held together in two ways:

(a) The intervertebral disc holds the bodies of adjacent vertebrae together. The intervertebral disc is a fibrous ring with a soft center. This disc allows the vertebral bodies to move on one another. This joint between the vertebral bodies is a plane-type joint.

(b) The various parts of adjacent vertebrae are held together by ligaments. A ligament is a dense FCT structure which extends from bone to bone. These ligaments extend along the vertebral column from the base of the skull all the way down to the coccyx.

(4) The spine has four curvatures in the adult human. In the cervical (neck) region and the lumbar (low back) region, the spine curves forward. In the thoracic (chest) region and the sacro-coccygeal (pelvic- sacrum and coccyx) region, the spine curves backwards.

(5) When one examines the back of a person by sight and feel (palpation), certain landmarks are observed.

(a) At the upper shoulder region in the midline, a knob can be seen and felt. This is the tip of the spinous process of the seventh cervical vertebra. Since this is the first vertebra from the top that can be easily palpated, this bony landmark is called the vertebra prominens (the "prominent vertebra").

(b) From the vertebra prominens down to the beginning of the sacrum, one can feel the tip of the spinous process of each vertebra.

b. The Thoracic (Rib) Cage. The rib cage (figure 4-5) forms a protective enclosure for the vital organs contained within the thorax (chest) such as the heart and lungs. It also allows the movements of breathing to take place.

(1) The sternum lies in the midline of the thorax anteriorly. It is made up of three parts: the manubrium at the top, the body as the main part, and the xiphoid process below. On the top of the manubrium is the jugular (sternal) notch, a common landmark. The junction between the manubrium and the body is a joint called the sternal angle. This sternal angle is an important landmark clinically because the second rib attaches to the sternum at this junction. It is just a matter of simple counting after identifying the second rib to know where you are on the thoracic wall.

(2) The rib cage consists of the 12 thoracic vertebrae, 12 pairs of ribs, and the sternum. Each rib is curved laterally from back to front. All 12 pairs of ribs are attached posteriorly to the thoracic vertebrae. The upper six pairs of ribs are attached directly to the sternum by their costal cartilages. The seventh through tenth pairs of ribs are attached indirectly to the sternum through their costal cartilages (by attaching to the costal cartilage of the rib above). Rib pairs 11 and 12 do not attach to the sternum. Instead, they are embedded in the trunk wall muscles.

c. The Skull. The skull (figure 4-6) is the bony framework (skeleton) of the head region. It has two major subdivisions: the cranium which encases and protects the brain and the facial skeleton which is involved with the beginnings of the digestive and respiratory systems. The special sense organs (eyes, ears, etc.) are included and protected within the skull.

(1) The bones of the cranium form a spherical case around the brain. With age, the sutures between the cranial bones become more solid. The cranium has a base with several openings for the passage of blood vessels and nerves. The vault (or calvaria) is made up of flat bones arching over and covering the brain.

(2) The facial skeleton consists of bones which surround the nose and the mouth. These are mainly flat and irregular bones. Bones of the facial skeleton also form part of the orbit of each eye.

(3) Certain bones of the skull have air-filled spaces called the paranasal sinuses.

(4) The upper jaw (maxilla) and the lower jaw (mandible) are parts of the facial skeleton which surround the mouth.

(5) The hyoid bone is located at the junction between the head and the neck. It is not articulated directly with the other bones. It is held in place--and moved around--by groups of muscles above and below. The root of the tongue is attached to its upper anterior surface. The larynx is suspended from its inferior surface. These three structures, together, form the hyoid complex. This complex is a functional unit for swallowing.

THE APPENDICULAR SKELETON

a. The appendicular skeleton is made up of the skeletal elements of the upper and lower members (often incorrectly referred to as the "extremities"). These members are appended (attached) to the axial skeleton.

b. The general pattern of construction of the upper and lower members is the same as follows:

(1) Girdle. The girdle is the actual attaching part. It attaches (appends) the limb (the member less the girdle) to the axial skeleton.

(2) Proximal limb segment. The proximal segment of the limb has a single long bone.

(3) Middle limb segment. The middle segment of the limb has two long bones parallel with each other.

(4) Distal limb segment. The distal segment of the limb is made up of many long and short bones. These bones are arranged into a five-rayed pattern--the digits.

 

 

 

 

 

c. See table 4-2 for the main bones of the upper and lower members.

PART	UPPER MEMBER	LOWER MEMBER
GIRDLE	PECTORAL GIRDLE (CLAVICLE AND SCAPULA)	PELVIC GIRDLE(PELVIC BONE--A FUSION OF ILIUM, PUBIS, AND ISCHIUM)
PROXIMAL SEGMENT	HUMERUS	FEMUR
MIDDLE SEGMENT	RADIUS ULNA	TIBIA FIBULA
DISTAL SEGMENT	CARPUS (8 WRIST BONES) METACARPALS (5) PHALANGES (5 DIGITS)	TARSUS (7 ANKLE BONES) METATARSALS (5) PHALANGES (5 DIGITS)