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if taken altogether, far more oxygen than the food and water taken into the body. Now, the only possible source whence the body can obtain oxygen, except from food and water, is the air which surrounds it. And careful investigation of the air which leaves the chamber in the imaginary experiment described above would show, not only that it has gained carbonic acid from the man, but that it has lost oxygen in equal or rather greater amount to him. 6. Thus, if a man is neither gaining nor losing weight, the sum of the weights of all the substances above enumerated which leaves the body ought to be exactly equal to the weight of the food and water which enter it, together with that of the oxygen which it absorbs from the air. And this is proved to be the case.

Hence it follows that a man in health, and "neither gaining nor losing flesh," is incessantly oxidating and wasting away, and periodically making good the loss. So that if, in his average condition, he could be confined in the scale-pan of a delicate spring balance, like that used for weighing letters, the scale-pan would descend at every meal, and ascend in the intervals, oscillating to equal distances on each side of the average position, which would never be maintained for longer than a few minutes. There is, therefore, no such thing as a sta- • tionary condition of the weight of the body, and what we call such is simply a condition of variation within narrow limits--a condition in which the gains and losses of the numerous daily transactions of the economy balance one another.

7. Suppose this diurnally-balanced physiological state to be reached, it can be maintained only so long as the quantity of the mechanical work done, and of heat, or other force evolved, remains absolutely unchanged.

Let such a physiologically-balanced man lift a heavy body from the ground, and the loss of weight which he would have undergone without that exertion will be immediately increased by a definite amount, which cannot be made good unless a proportionate amount of

1 Fresh country air contains in every 100 parts nearly 21 of oxygen and 79 of nitrogen gas, together with a small fraction of a part of carbonic acid, a minute uncertain proportion of ammonia, and a variable quantity of watery vapour. (See Lesson IV. § 11.)

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 energy the body gives out, 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 vertebra. 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 incloses, 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

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A. A diagrammatic section of the human body taken vertically through the median plane. C.S. the cerebro-sp.nal 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 ab; letters

as before.

C. A transverse section taken along the line cd; letters as before.

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axis (Fig. 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 esophagus) 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 tube contains the alimentary canal, the sympathetic nervous system, the heart, and the lungs, besides other organs.

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

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