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Human Anatomy in Full Color

Dover Publications, Inc.

The Skeleton

The 206 bones in the skeleton form the framework of the body, accounting for one seventh of an adult male's weight. The 80 bones of the head, face, neck and trunk are called the axial skeleton. The appendicular skeleton, comprising the limbs or appendages, is made up of 32 bones in each adult upper limb and 31 in each lower limb. Sesamoid bones are connected to a tendon rather than to another bone; everybody has at least two—the patella (kneecap) of each leg—but some people have them in other sites such as the foot or over the first joint in the thumb. (If present, these are extra bones and are always small.) The largest bone in the body is the femur, the smallest are the ossicles in the middle ear.

Bones have several important functions. They are the support system of the body for soft tissues and anchorage points for muscles. To allow movement of the body, individual bones must be able to move in relationship to one another. This happens at joints, where the bones articulate with each other, the surfaces that touch being termed articular surfaces.

Groups of bones form protective cages: The skull protects the brain, and the lungs and heart are enclosed by the bones of the vertebral column (spine), ribs and sternum (breastbone). There are 12 pairs of ribs; the lower two pairs of ribs "float" and are not connected to the "gristle" or cartilage that composes the junction between the ribs and breastbone. In the pelvis the bone arrangement protects vital organs of the lower abdomen, but the female pelvis is wider than the male to allow space for a baby to pass through when it is born. The sacrum ends in the coccyx, which is a residual tail.

Inside bones, red and white blood cells and platelets are manufactured and essential elements such as calcium and phosphate are stored.

Bones & Joints

There are three kinds of bone—long (e.g., thigh and arm); flat (the shoulders, skull, ribs and pelvis) and irregular (the spinal column, ankle and wrist, although the last are sometimes called short bones). Bones are covered by a tough sheath, the periosteum, which contains many small nerve endings. This is why a blow on a bone such as the shin is so painful. The two types of bone, spongy (cancellous) and cortical, are composed of Haversian canals carrying nerves, blood vessels and lymphatics, surrounded by plates of bone and spaces containing bone cells—osteoblasts and osteocytes, which produce bone, and osteoclasts, which reabsorb it. Even in adults, 15 percent of bone may be remodeled in a year. Spongy bone has the appearance of a sponge as the canals are larger and the spaces contain bone marrow, comprised of blood and fat cells. In long bones, red blood cells are manufactured in the red bone marrow, which predominates in childhood, but in adults, this is replaced mainly by yellow, fatty marrow in the medullary cavity.

Bones are formed from calcium and phosphorus derived from diet and combined to form calcium phosphate. Long bones grow as new bone is deposited by the layer beneath the periosteum and at bands of cartilage, called epiphyseal plates, near the articular surfaces, until full growth is reached between the ages of 18 and 25, when bone replaces this cartilage. The age at which a bone stops growing varies from one bone to another.

A joint is formed where two bones meet. The surfaces on which they move on one another are articular surfaces. They are covered in articular cartilage, which acts as a cushion. There are three types of joints. Fibrous joints are fixed and immovable, such as the bones of the skull. Cartilaginous joints are slightly movable joints in which a plate of fibrocartilage separates the bones, as in the breastbone (sternum) and the vertebral bodies of the spinal column. Synovial joints are freely movable and lined by a membrane producing a lubricant, synovial fluid. There are seven varieties of synovial joint. A hinge joint, like the elbow, moves up and down like the lid of a box. A pivot joint allows rotation only, as in the radius and ulna bones at the elbow, or the second bone in the neck. In a plane joint the surfaces are flat, as in the wrist and foot. Ball and socket joints, such as the shoulder and hip, can move in all directions. Condyloid joints are modified ball-and-socket joints that cannot move upward, such as the fingers and knuckle joints. An ellipsoid joint is another modified ball-and-socket joint in the wrist involving the carpal and radius bones, capable of all movements except rotation. A saddle joint, such as the thumb, moves on two axes at right angles to each other. Joints are surrounded by a capsule, which is further strengthened by ligaments.

The Skull

The facial appearance of any person depends partly on the underlying size and shape of the skull, which is the skeleton of the head and face, and the muscles attached to it. Only the mandible of the jaw moves; the other 21 bones have fused together by adulthood. Eight of these bones are paired, five are unpaired and the areas of fusion are known as sutures. The infant skull differs from the adult: It is larger in comparison with the rest of the body. Infant sutures are very wide at birth and allow "molding" or shaping of the skull for a damage-free passage through the birth canal. During the first 18 months of life the gaps can be felt in certain places, especially the diamond-shaped anterior fontanelle at the junction of the frontal and parietal bones. Another space—between the occipital and parietal bones—can be felt during a baby's first two months. Sometimes, in disease, the bones fuse together too soon or stay apart too long.

The only movable joint in the head is the temporomandibular, where the mandible (lower jaw) forms a hinge with the temporal bone. It can move in three planes, placing the jaw up, down, backward and forward as well as side to side. There are three specialized openings in the skull for ears, eyes and nose. The eye nestles in the orbital cavity, which is protected by a bony rim. Blows to the eye may fracture the zygomatic bone and maxilla (part of the upper jaw) rather than injure the eye itself. The nose has a root and roof of bone but the lower third—the nostrils—is soft and composed of cartilage. When a nose bleeds, the only way to compress the blood vessels and stop the flow is by pinching this soft part. In order to lighten the facial bones, parts are hollowed out into air sinuses (frontal and maxillary), which are common sites of infection (sinusitis). The largest cavity in the skull contains and protects the brain. The spinal column leaves the skull through the foramen magnum.

Both jaws carry an equal number of teeth. There are 20 milk (deciduous) teeth in a child. These appear in a set sequence from about six months and begin to be replaced by permanent teeth from six years. By 17-21 years the complete set of 32 has appeared with the presence of the third molar or "wisdom" tooth. Each quarter of an adult jaw has two incisors for cutting, one canine for tearing, and two premolars (two cusps) together with three molars (three or four cusps) for grinding. Every tooth has a root in a socket under the gum, a small neck at gum level and a crown, varying in shape according to its function. Crowns are composed of exceptionally hard dentine covered in shining, protective enamel. The tooth cavity contains blood vessels and nerves, damage to which gives rise to a toothache.

Muscles

The muscles that cover the skeleton, contracting at will to make movements of bone possible, are called voluntary muscles. They also cause movements of overlying tissues, as in the face, where they create the expression of feelings, as shown by smiles and frowns. Skeletal muscle constitutes about 42 percent of a male body and 36 percent of a female body. There are three types of muscles. Striated muscle reveals patterns when viewed under a microscope. All voluntary muscles are striated. Smooth, or unstriated, muscle is found in the walls of blood vessels, the alimentary tract and the ducts of glands. Heart muscle is a specialized type of striated muscle, contracting automatically at a rate regulated by its nerve supply.

A muscle fiber can vary in size from fractions of an inch (several millimeters) to over one foot (30 centimeters). Skeletal muscle consists of bundles of fibers held together by connective tissue and enclosed in a tough fibrous sheath called fascia, which is usually connected at one or both ends to bundles of white fibrous tissue—a tendon. Tendons anchor muscles to bones and joints. In order to move bones and joints, a muscle must be fixed at its origin, which is nearer the inner aspect of the body and the upper part of a limb. The other end (the insertion) is attached to a point away from the center of the body and toward the end of a limb. Some attachments are complex fibrous structures. When a muscle contracts, the origin remains stationary and the insertion moves. Muscles must cross the joint that they move; some pass over more than one joint, for example, the biceps, which passes over the shoulder and the elbow. When a muscle contracts, it shortens and pulls, but cannot push. As a contraction passes away, the muscle becomes soft and longer. However, movement cannot take place unless other muscles, having the opposite action, relax—these paired muscles are called antagonists.

Energy for movements comes from glycogen, the chief carbohydrate storage material in animals, which is found in the muscles and the liver. Glycogen is broken down into carbon dioxide and water with the release of energy through a process of oxidation, in which oxygen is consumed. During leisurely activity, enough oxygen is available, but during violent exercise, there is often not enough oxygen, and lactic acid is produced instead of carbon dioxide. This buildup of lactic acid in muscle tissue gives rise to cramps and fatigue.

The Spine

The spine, also known as the spinal or vertebral column, stretches from the base of the skull to the coccyx and is composed of bones called vertebrae. Disks of fibrocartilage, known as intervertebral disks, separate the body of each vertebra with ligaments in front and behind to keep the spinal column together in its characteristic S-shaped curve. The column is flexible and maintains the upright posture of the body. A typical vertebra has a thick, strong body, facing frontward with an arch of bone pointing backward and enclosing the spinal cord in the space called the vertebral foramen. The seven cervical vertebrae in the neck are the smallest, the first two allow rotation of the skull. They are so specialized in shape that they have their own names—atlas and axis. The 12 thoracic vertebrae increase in size from top to bottom and have long spines and extra facets to articulate with the ribs. The top eight join with two pairs of ribs—its own and the one below. The lower four join with their own ribs. Because they keep the upper and lower parts of the body together, the five lumbar vertebrae have very thick, strong bodies with heavy spinous processes for the attachment of muscles. The sacrum, which gives stability to the pelvis between the hips, is composed of five vertebrae fused to form a triangular bone. The coccyx, also triangular, is composed of four rudimentary vertebrae which articulate with, or sometimes even fuse with, the sacrum.

The spinal cord extends from the base of the skull to the first lumbar vertebra—an average distance of 17½ inches (45 centimeters). It branches into pairs of nerves, each having an anterior (front) motor root, which initiates activity, and a posterior sensory root to the back of the cord. The two roots join to form spinal nerves. There are 31 pairs made up of eight to the neck or cervical region, 12 to the chest (thoracic nerves), five to the lower back (lumbar region), five to the sacrum (sacral nerves) and one to the coccyx. Some spinal nerves join up to form a plexus: cervical 1-4 go to the neck and shoulder with the phrenic nerve serving the diaphragm; cervical 5 to thoracic 1 form the brachial plexus which becomes the radial, median and ulnar nerves in the forearm. Lumbar 1-4 nerves form the femoral nerve, which runs down the front of the thigh, and lumbar nerves 4 and 5 and sacral 1-3 form the sciatic nerve, the largest in the body, extending down the back of the leg, dividing above the knee into peroneal and tibial nerves. The sciatic nerve is commonly damaged in back injuries.

The Brain

The brain is in three parts—fore, mid and hind—protected by three membranes: the dura mater, arachnoidea and pia mater, collectively called the meninges. The largest part of the forebrain is the cerebrum, divided into two cerebral hemispheres by a huge fissure. Each half—left and right—has sensory areas for receiving messages and motor areas for sending out messages, normally to the opposite side of the body. The outer layer of the cerebrum is the cerebral cortex, composed of nerve cells (gray matter). The tissue below appears white because it contains nerve fibers. The hemispheres are joined by a band of nerve fibers called the corpus callosum. The rest of the forebrain contains the egg-shaped thalamus, which integrates the sensory, motor and emotional processes of the body; the hypothalamus, which regulates the autonomic system (nerves controlling the heart, glands and smooth muscles) and the pituitary gland below it by secreting hormones into the bloodstream. The midbrain is small—about ¾ inch (two centimeters) long—connecting the forebrain with the hindbrain (principally the cerebellum), which coordinates the accuracy of muscle activity and maintains the posture of the body. The brain stem consists of the pons, carrying communication fibers to and from the cerebellum and the medulla oblongata. The medulla oblongata contains nerve centers essential to life, which control the heart and respiration. The twelve pairs of cranial nerves supply the organs of the head and neck, extending via the tenth (vagus) nerve to the chest, heart and abdomen.

Nervous System

The nervous system is the communications network of the body. It has two parts—the central nervous system, consisting of the brain and spinal cord, and the peripheral nervous system, radiating from the spinal cord to muscles, internal organs and skin. Sensations from the environment outside the body, internal organs and other tissues are interpreted and the appropriate reaction is registered by the brain. Nerve fibers form the bulk of the brain, spinal cord and peripheral nerves. Every nerve fiber consists of a thread-like axon, which may range from a few centimeters to many centimeters in length. The fiber may or may not be covered by a fatty myelin sheath, produced by Schwann cells. Nerve fibers are classified as myelinated or nonmyelinated. There are gaps in the myelin sheath called nodes of Ranvier where nutrients can be absorbed and waste products excreted. Disease can occur when the myelin covering is lost. Myelinated fibers form the white matter of the brain and spinal cord, whereas nerve cell bodies comprise the gray matter. The basic unit of the nervous system is a neuron, consisting of a nerve cell with several branching processes (dendrites) which receive nerve impulses (waves of changing electrical energy). These pass down myelinated fibers faster—at over 328 feet (100 meters) per second—than through nonmyelinated fibers. Impulses are passed from the end of one fiber to another via a synapse or to a muscle fiber at a neuromuscular junction.

The Skin & Hair

The skin—strong enough to withstand everyday knocks, yet supple enough to allow movement and varying in thickness from 1/16 inch (5 millimeters) on the soles of the feet to less than 1/32 inch (0.5 millimeters) on the eyelids—envelops the whole body. Accounting for 15 percent of adult body weight, skin is the body's largest organ. Its functions include sensing changes in the environment, controlling body temperature, acting as a waterproof barrier or a screen against damaging radiation from the sun and protecting underlying tissues from infection. Skin has two principal layers—the dermis and epidermis. The underlying dermis contains fat cells, blood and lymphatic vessels, sweat and sebaceous glands, elastic fibers and Meissner's corpuscles, which detect touch and are more plentiful on tips of fingers, toes and lips. Oil from sebaceous glands keeps the skin surface supple, maintaining its texture and waterproof properties. The epidermis consists of layer upon layer of cells in varying stages of destruction—flat and without nuclei (stratum corneum), granular (stratum granulosum) and those with spines (stratum spinosum)—all derived from the base layer (stratum basale). The outer layers of skin, containing keratin, a tough protein also present in nails and hair, are continually shed and replaced by those below. The skin is colored by various pigments in the epidermis, including melanin, which changes with exposure to sunlight, giving a tan. Sweat glands help to keep body temperature constant by secreting sweat, which evaporates to cool the skin. Hair shafts grow from follicles. After about 1000 days a hair shaft is shed, to be replaced. About 100 are shed each day.

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Excerpted from Human Anatomy in Full Color by John W. Harcup, John Green. Copyright © 1996 John W. Harcup. Excerpted by permission of Dover Publications, Inc..
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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