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Human Anatomy Illustrations

Learn about the ten systems in the human body.

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Anatomy (Greek anatomē, “dissection”), branch of natural science dealing with the structural organization of living things. It is an old science, having its beginnings in prehistoric times. For centuries anatomical knowledge consisted largely of observations of dissected plants and animals. The proper understanding of structure, however, implies a knowledge of function in the living organism. Anatomy is therefore almost inseparable from physiology, which is sometimes called functional anatomy. As one of the basic life sciences, anatomy is closely related to medicine and to other branches of biology.

It is convenient to subdivide the study of anatomy in several different ways. One classification is based on the type of organisms studied, the major subdivisions being plant anatomy (see Plant) and animal anatomy. Animal anatomy is further subdivided into human anatomy (see below) and comparative anatomy, which seeks out similarities and differences among animal types (see Animal). Anatomy can also be subdivided into biological processes—for example, developmental anatomy, the study of embryos, and pathological anatomy, the study of diseased organs. Other subdivisions, such as surgical anatomy and anatomical art, are based on the relationship of anatomy to other branches of activity under the general heading of applied anatomy. Still another way to subdivide anatomy is by the techniques employed—for example, microanatomy, which concerns itself with observations made with the help of the microscope (see the section below on the history of anatomy).




The human body operates through systems that are summarized below. This encyclopedia contains separate articles on each of the systems and organs mentioned to which the reader is referred for more comprehensive discussions. References to articles discussing the senses and perception are listed in Sense Organs.



Musculoskeletal System

Cross Section of a Bone

Bone marrow, the soft, pulpy tissue that fills bone cavities, contains a network of blood vessels and fibers surrounded by fat and blood-producing cells. In children, the cells that give rise to blood cells can be found throughout the marrow. In adults, these cells are found mostly in the red marrow of the bones of the chest, hips, back, skull, and of the upper arms and legs. The marrow in the long shafts of bones gradually loses its ability to manufacture blood. This marrow, which is dominated by fat cells and takes on a yellowish color, is called yellow marrow. This cross section of a long bone shows yellow bone marrow in the shaft of a long bone.

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The human skeleton consists of more than 200 bones bound together by tough and relatively inelastic connective tissues called ligaments. The different parts of the body vary greatly in their degree of movement. Thus, the arm at the shoulder is freely movable, whereas the knee joint is definitely limited to a hingelike action. The movements of individual vertebrae are extremely limited; the bones composing the skull are immovable. Movements of the bones of the skeleton are effected by contractions of the skeletal muscles, to which the bones are attached by tendons. These muscular contractions are controlled by the nervous system. See Muscle; Skeleton.



Nervous System

Nervous System Organization

The nervous system is composed of the central nervous system and the peripheral nervous system. The central nervous system, which includes the brain and spinal cord, processes and coordinates all incoming sensory information and outgoing motor commands, and it is also the seat of complex brain functions such as memory, intelligence, learning, and emotion. The peripheral nervous system includes all neural tissue outside of the central nervous system. It is responsible for providing sensory, or afferent, information to the central nervous system and carrying motor, or efferent, commands out to the body’s tissues. Voluntary motor commands, such as moving muscles to walk or talk, are controlled by the somatic nervous system, while involuntary motor commands, such as digestion and heart beat, are controlled by the autonomic nervous system. The autonomic nervous system is further divided into two systems. The sympathetic nervous system, sometimes called the “fight or flight” system, increases alertness, stimulates tissue, and prepares the body for quick responses to unusual situations. In contrast, the parasympathetic nervous system, sometimes called the “rest and repose” system, conserves energy and controls sedentary activities, such as digestion.

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The nervous system has two divisions: the central nervous system and the peripheral nervous system. The central nervous system includes the brain and spinal cord. It processes incoming sensory information and sends outgoing motor commands. The peripheral nervous system includes all neural tissue outside the central nervous system. It is divided into motor and sensory systems. Impulses go to the central nervous system through sensory nerves and are carried away from it by the motor nerves. The motor system is further divided into the somatic (or skeletal) nervous system and the autonomic nervous system. The somatic, or skeletal, motor system allows voluntary control over skeletal muscle with a few exceptions. The autonomic nervous system is largely involuntary and controls cardiac and smooth muscles and glands.

Neural Organization

Nervous systems range in complexity from the jellyfish’s network of nerve cells to the central and peripheral systems of humans. Common to many animals is the nervous structure of the earthworm, which consists of a cerebral ganglion, a main nerve cord, and branching pairs of lateral nerves. In some cases, as in insects, the cerebral ganglion acts as a primitive brain, controlling and coordinating various basic functions.

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The autonomic nervous system has three divisions: the enteric, the sympathetic, and the parasympathetic. The enteric nervous system is a system of nerves in the gastrointestinal tract, pancreas, and gallbladder that influences all digestive processes. The enteric system operates without input from the brain or spinal cord.

Human Brain

The human brain has three major structural components: the large dome-shaped cerebrum (top), the smaller somewhat spherical cerebellum (lower right), and the brainstem (center). Prominent in the brainstem are the medulla oblongata (the egg-shaped enlargement at center) and the thalamus (between the medulla and the cerebrum). The cerebrum is responsible for intelligence and reasoning. The cerebellum helps to maintain balance and posture. The medulla is involved in maintaining involuntary functions such as respiration, and the thalamus acts as a relay center for electrical impulses traveling to and from the cerebral cortex. Lack of blood flow to any part of the brain results in a stroke, permanent damage that interferes with the functions of the affected part of the brain.

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The sympathetic and parasympathetic divisions may operate together or in opposition. Many, but not all, of the muscles and glands that distribute nerve impulses to the larger interior organs have both sympathetic and parasympathetic nerve systems. In such cases the two divisions may exert opposing effects. Thus, the sympathetic system increases heartbeat, and the parasympathetic system decreases heartbeat. The two nervous systems are not always in opposition, however. For example, both nerve supplies to the salivary glands excite the cells of secretion. Furthermore, a single division of the autonomic nervous system may both stimulate and inhibit, as in the sympathetic supply to the blood vessels of skeletal muscle. Finally, the sweat glands, the muscles that cause involuntary erection or bristling of the hair, the smooth muscle of the spleen, and the blood vessels of the skin and skeletal muscle are actuated only by the sympathetic division.

Cranial Nerves

Whereas most major nerves emerge from the spinal cord, the 12 pairs of cranial nerves project directly from the brain. All but 1 pair relay motor or sensory information (or both); the tenth, or vagus nerve, affects visceral functions such as heart rate, vasoconstriction, and contraction of the smooth muscle found in the walls of the trachea, stomach, and intestine.

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Voluntary movement of head, limbs, and body is caused by nerve impulses arising in the motor area of the cortex of the brain and carried by cranial nerves or by nerves that emerge from the spinal cord to connect with skeletal muscles. The reaction involves both excitation of nerve cells stimulating the muscles involved and inhibition of the cells that stimulate opposing muscles. A nerve impulse is an electrical change within a nerve cell or fiber; measured in millivolts, it lasts a few milliseconds and can be recorded by electrodes.

How a Reflex Works

Reflexes are involuntary reactions by our nervous systems. They activate our muscles in situations where our brain would take too long to react. Voluntary movements occur in response to messages from the brain.

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Movement may occur also in direct response to an outside stimulus; thus, a tap on the knee causes a jerk, and a light shone into the eye makes the pupil contract. These involuntary responses are called reflexes. Various nerve terminals called receptors constantly send impulses into the central nervous system. These are of three classes: exteroceptors, which are sensitive to pain, temperature, touch, and pressure; interoceptors, which react to changes in the internal environment; and proprioceptors, which respond to variations in movement, position, and tension. These impulses terminate in special areas of the brain, as do those of special receptors concerned with sight, hearing, smell, and taste.

Muscular contractions do not always cause actual movement. A small fraction of the total number of fibers in most muscles are usually contracting. This serves to maintain the posture of a limb and enables the limb to resist passive elongation or stretch. This slight continuous contraction is called muscle tone.