CHAPTER II - GENERAL VIEW OF THE BODY
External Divisions.—Examined from the outside, the body presents certain parts, or divisions, familiar to all. The main, or central, portion is known as the trunk, and to this are attached the head, the upper extremities, and the lower extremities. These in turn present smaller divisions which are also familiar. The upper part of the trunk is known as the thorax, or chest, and the lower part as the abdomen. The portions of the trunk to which the arms are attached are the shoulders, and those to which the legs are joined are the hips, while the central rear portion between the neck and the hips is the back. The fingers, the hand, the wrist, the forearm, the elbow, and the upper arm are the main divisions of each of the upper extremities. The toes, the foot, the ankle, the lower leg, the knee, and the thigh are the chief divisions of each of the lower extremities. The head, which is joined to the trunk by the neck, has such interesting parts as the eyes, the ears, the nose, the jaws, the cheeks, and the mouth. The entire body is inclosed in a double covering, called the skin, which protects it in various ways.
The Tissues.—After examining the external features of the body, we naturally inquire about its internal structures. These are not so easily investigated, and much which is of interest to advanced students must be omitted from an elementary course. We may, however, as a first step in this study, determine what kinds of materials enter into [pg 005]the construction of the body. For this purpose the body of some small animal should be dissected and studied. (See observation at close of chapter.) The different materials found by such a dissection correspond closely to the substances, called tissues, which make up the human body. The main tissues of the body, as ordinarily named, are the muscular tissue, the osseous tissue, the connective tissue, the nervous tissue, the adipose tissue, the cartilaginous tissue, and the epithelial and glandular tissue. Most of these present different varieties, making all together some fifteen different kinds of tissues that enter into the construction of the body.[2]
General Purposes of the Tissues.—The tissues, first of all, form the body. As a house is constructed of wood, stone, plaster, iron, and other building materials, so is the body made up of its various tissues. For this reason the tissues have been called the building materials of the body.
In addition to forming the body, the tissues supply the means through which its work is carried on. They are thus the working materials of the body. In serving this purpose the tissues play an active rôle. All of them must perform the activities of growth and repair, and certain ones (the so-called active tissues) must do work which benefits the body as a whole.
Purposes of the Different Tissues.—In the construction of the body and also in the work which it carries on, the different tissues are made to serve different purposes. The osseous tissue is the chief substance in the bony framework, or skeleton, while the muscular tissue produces the different movements of the body. The connective[pg 006] tissue, which is everywhere abundant, serves the general purpose of connecting the different parts together. Cartilaginous tissue forms smooth coverings over the ends of the bones and, in addition to this, supplies the necessary stiffness in organs like the larynx and the ear. The nervous tissue controls the body and brings it into proper relations with its surroundings, while the epithelial tissue (found upon the body surfaces and in the glands) supplies it with protective coverings and secretes liquids. The adipose tissue (fat) prevents the too rapid escape of heat from the body, supplies it with nourishment in time of need, and forms soft pads for delicate organs like the eyeball.
Properties of the Tissues.—If we inquire how the tissues are able to serve such widely different purposes, we find this answer. The tissues differ from one another both in composition and in structure and, on this account, differ in their properties.[3] Their different properties enable them to serve different purposes in the body. Somewhat as glass is adapted by its transparency, hardness, and toughness to the use made of it in windows, the special properties of the tissues adapt them to the kinds of service which they perform. Properties that adapt tissues to their work in the body are called essential properties. The most important of these essential properties are as follows:
1. Of osseous tissue, hardness, stiffness, and toughness. 2. Of muscular tissue, contractility and irritability. 3. Of nervous tissue, irritability and conductivity. 4. Of cartilaginous tissue, stiffness and elasticity. 5. Of connective tissue, toughness and pliability. 6. Of epithelial tissue, ability to resist the action of external forces and power to secrete.