The flexible and soft part, which everywhere covers organs and muscles, is composed of a layer of fat to preserve the warmth, as fat is a non-conductor, and an outer covering of skin.
This framework is exquisitely adapted to give strong protection to the vital parts so that they cannot readily be injured; and the whole of the organs are so arranged and stowed away that a perfect human body is a beautiful object full of symmetry and graceful curves and lines.
Divisions of the Body.—If we divide the body into six parts—four limbs, trunk, and head and neck—we find each part contains about thirty bones (counting the ribs in pairs) there being about two hundred in the entire body.
The height of the body depends mainly on the length of the bones of the lower limbs.
Everything in Pairs.—In the body almost everything is paired, right and left, giving it symmetry. There are but five central bones: two in the head, one in the throat, and the breastbone and backbone (or spine); and there are but five single muscles, all the rest—out of many hundreds—being in pairs. In the interior, where economy rather than symmetry is required, it is not so; there being as many single organs as there are double.
The Body Viewed as a Machine.—A favorite way of looking at the body as a whole is to regard it as an anatomical machine. In this view the body has an internal skeleton, of which the chief feature is the central axis or backbone.
Considering the skull and backbone as one, the body may be said to be built up of two tubes. The smaller posterior or neural tube includes the cavity of the skull and the vertebral canal. Within this tube is lodged the nervous center, or engine, of the body. The anterior, or body, tube is much larger, consisting of the face above, and the neck and trunk below, and it contains the four nutritive systems of life, so that the whole body in section is like an eight with the lower circle immensely exaggerated. The limbs, of course, are not tubular, and merely form part of the machinery.
Adopting the simile of the human engine and boiler and machinery, we see that the limbs, etc., are the machinery; the posterior tube the engines and force that move them; and the anterior tube the human boiler that generates the force. This boiler, like one in a steam engine, has an upper and lower part. The upper part is where the steam is generated (in lungs) and sent to the engine (the brain) by the heart. The lower part is where the fuel is burned (the stomach) and the ashes and refuse drop through (the intestines). So that the analogy between the two is close and striking.
Centers of Control.—There are two distinct seats of government in the human body: the one in the upper brain, or cortex, the other principally in the very center of the human body. That in the upper brain, or cortex, is the human will and the conscious mind. It has absolute control given to it over the animal part of the human life—that is, over the part that consists in the using of force, which includes the nervous and locomotor systems, and the special senses.
Nutritive Systems.—The other government, situated in the lower part of the brain and spinal cord and in the center of the body—in front of the spine and behind the stomach—is of an entirely different order. To put this more plainly: The four systems that lie in the body—digestive, circulatory, respiratory, and excretory—may be termed the nutritive systems, being designed for the maintenance and storage of life-forces. They are almost entirely under the control of the involuntary nerve centers, and have full and undisputed sway over life itself—that is, over the generating and storing of vital force, rather than over its usage.