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extra food be supplied to him. Let the temperature of the air fall, and the same result will occur, if his body remains as warm as before.
On the other hand, diminish his exertion and lower his production of heat, and either he will gain weight, or some of his food will remain unused.
Thus, in a properly nourished man, a stream of food is constantly entering the body in the shape of complex compounds containing comparatively little oxygen; as constantly, the elements of the food (whether before or after they have formed part of the living substance) are leaving the body, combined with more oxygen. And the incessant breaking down and oxidation of the complex compounds which enter the body are definitely proportioned to the amount of force the body cxcrts, whether in the shape of heat or otherwise; just in the same way as the amount of work to be got out of a steam-engine, and the amount of heat it and its furnace give off, bear a strict proportion to its consumption of fuel.
8. From these general considerations regarding the nature of life, considered as physiological work, we may turn for the purpose of taking a like broad survey of the apparatus which does the work. We have seen the general performance of the engine, we may now look at its build.
The human body is obviously separable into head, trunk, and limbs. In the head, the brain-case or skull is distinguishable from the face. The trunk is naturally divided into the chest or thorax, and the belly or abdomen. Of the limbs there are two pairs-the upper, or arms, and the lower, or legs; and legs and arms again are subdivided by their joints into parts which obviously exhibit a rough correspondence--thigh and upper arm, leg and fore-arm, ankle and wrist, fingers and toes, plainly answering to one another. And the two last, in fact, are so similar that they receive the same name of digits; while the several joints of the fingers and toes have the common denomination of phalanges.
The whole body thus composed (without the viscera) is seen to be bilaterally symmetrical; that is to say, if it were split lengthways by a great knife, which should be made to pass along the middle line of both the dorsal and
ventral (or back and front) aspects, the two halves would almost exactly resemble one another.
9. One-half of the body, divided in the manner described (Fig. 1, A), would exhibit, in the trunk, the cut faces of thirty-three bones, joined together by a very strong and tough substance into a long column, which lies much nearer the dorsal (or back) than the ventral (or front) aspect of the body. The bones thus cut through are called the bodies of the vertebræ. They separate a long, narrow canal, called the spinal canal, which is placed upon their dorsal side, from the spacious chamber of the chest and abdomen, which lies upon their ventral side. There is no direct communication between the dorsal canal and the ventral cavity.
The spinal canal contains a long white cord-the spinal cord-which is an important part of the nervous system. The ventral chamber is divided into the two subordinate cavities of the thorax and abdomen by a remarkable, partly fleshy and partly membranous, partition, the diaphragm (Fig. 1, D), which is concave towards the abdomen, and convex towards the thorax. The alimentary canal (Fig. 1, Al.) traverses these cavities from one end to the other, piercing the diaphragm. So does a long double series of distinct masses of nervous substance, which are called ganglia, are connected together by nervous cords, and constitute the so-called sympathetic (Fig. 1, Sy.). The abdomen contains, in addition to these parts, the two kidneys, one placed against each side of the vertebral column, the liver, the pancreas or sweetbread," and the spleen. The thorax encloses, besides its segment of the alimentary canal and of the sympathetic, the heart and the two lungs. The latter are placed one on each side of the heart, which lies nearly in the middle of the thorax.
Where the body is succeeded by the head, the uppermost of the thirty-three vertebral bodies is followed by a continuous mass of bone, which extends through the whole length of the head, and, like the spinal column, separates a dorsal chamber from a ventral one. The dorsal chamber, or cavity of the skull, opens into the spinal canal. It contains a mass of nervous matter called the brain, which is continuous with the spinal cord, the brain and the spinal cord together constituting what is termed the cerebro-spinal
A. A diagrammatic section of the human body taken vertically through the median plane. C.S. the cerebro-spinal nervous system; N, the cavity of the nose; M, that of the mouth; Al. Al. the alimentary canal represented as a simple straight tube; H, the heart; D, the diaphragm; Sy. the sympathetic ganglia.
B. A transverse vertical section of the head taken along the line a b; letters as before.
C. A transverse section taken along the line c d; letters as before.
axis (Fig. 1, C.S., C.S.). The ventral chamber, or cavity of the face, is almost entirely occupied by the mouth and pharynx, into which last the upper end of the alimentary canal (called gullet or œsophagus) opens.
10. Thus, the study of a longitudinal section shows us that the human body is a double tube, the two tubes being completely separated by the spinal column and the bony axis of the skull, which form the floor of the one tube and the roof of the other. The dorsal tube contains the cerebro-spinal axis; the ventral, the alimentary canal, the sympathetic nervous system, and the heart, besides other
Transverse sections, taken perpendicularly to the axis of the vertebral column, or to that of the skull, show still more clearly that this is the fundamental structure of the human body, and that the great apparent difference between the head and the trunk is due to the different size of the dorsal cavity relatively to the ventral. In the head the former cavity is very large in proportion to the size of the latter (Fig. 1, B); in the thorax, or abdomen, it is very small (Fig. 1, C).
The limbs contain no such chambers as are found in the body and the head; but, with the exception of certain branching tubes filled with fluid, which are called bloodvessels and lymphatics, are solid or semi-solid, throughout.
II. Such being the general character and arrangement of the parts of the human body, it will next be well to consider into what constituents it may be separated by the aid of no better means of discrimination than the eye and the anatomist's knife.
With no more elaborate aids than these, it becomes easy to separate that tough membrane which invests the whole body, and is called the skin, or integument, from the parts which lie beneath it. Furthermore, it is readily enough ascertained that this integument consists of two portions a superficial layer, which is constantly being shed in the form of powder or scales, composed of minute particles of horny matter, and is called the epidermis; and the deeper part, the dermis, which is dense and fibrous (Fig. 32). The epidermis, if wounded, neither gives rise to pain nor bleeds. The dermis, under like circumstances, is very tender, and bleeds freely. A practical
distinction is drawn between the two in shaving, in the course of which operation the razor ought to cut only epidermic structures; for if it go a shade deeper, it gives rise to pain and bleeding.
The skin can be readily enough removed from all parts of the exterior, but at the margins of the apertures of the body it seems to stop, and to be replaced by a layer which is much redder, more sensitive, bleeds more readily, and which keeps itself continually moist by giving out a more or less tenacious fluid, called mucus. Hence, at these apertures, the skin is said to stop, and to be replaced by mucous membrane, which lines all those interior cavities, such as the alimentary canal, into which the apertures open. But, in truth, the skin does not really come to an end at these points, but is directly continued into the mucous membrane, which last is simply an integument of greater delicacy, but consisting fundamentally of the same two layers, a deep, fibrous layer, containing blood-vessels and nerves, and a superficial, insensible, and bloodless one, now called the epithelium. Thus every part of the body might be said to be contained between the walls of a double bag, formed by the epidermis, which invests the outside of the body, and the epithelium, its continuation, which lines the internal cavities.
12. The dermis, and the deep, sanguine layer, which answers to it in the mucous membranes, are chiefly made up of a filamentous substance, which yields abundant gelatine on being boiled, and is the matter which tans when hide is made into leather. This is called areolar, fibrous, or, better, connective tissue.1 The last name is the best, because this tissue is the great connecting medium by which the different parts of the body are held together. Thus it passes from the dermis between all the other organs, ensheathing the muscles, coating the bones and cartilages, and eventually reaching and entering into the mucous membranes. And so completely and thoroughly does the connective tissue permeate almost all parts of the body, that if every other tissue could be dissected away, a complete model of all the organs would be left composed of this tissue. Connective tissue varies very
1 Every such constituent of the body, as epidermis, cartilage, or muscle, is called a "tissue." (See Lesson XII.)