Anatomy

I

Introduction

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 on the history of anatomy).

II

Human Anatomy

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

A

Musculoskeletal System

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 hinge-like 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.

B

Nervous System

The nervous system has two divisions: the somatic, which allows voluntary control over skeletal muscle, and the autonomic, which is involuntary and controls cardiac and smooth muscle and glands. The autonomic nervous system has three divisions: the sympathetic, the parasympathetic, and the enteric nervous system. Many, but not all, of the muscles and glands that distribute nerve impulses to the larger interior organs possess a double nerve supply; in such cases the sympathetic and parasympathetic divisions may exert opposing effects. Thus, the sympathetic system increases heartbeat, and the parasympathetic system decreases heartbeat. The two nervous systems are not always antagonistic, 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 excite and inhibit a single effector, 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. The enteric division is essentially functionally independent from the other parts of the autonomic nervous system. It enervates the gastrointestinal tract.

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 those that emerge from the spinal cord to reach 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 fibre; measured in millivolts, it lasts a few milliseconds and can be recorded by electrodes.

Movement may occur also in direct response to an outside stimulus; thus, a tap on the knee causes a jerking action, 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 vision, hearing, smell, and taste.

Muscular contractions do not always cause actual movement. A small fraction of the total number of fibres 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.