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is breathed out of the man, and that which is given off from his skin, will exhibit clouds of vapour; which vapour, therefore, is derived from the body.

After the expiration of the hour during which the experiment has lasted, let the man be released and weighed He will be found to have lost weight.

once more.

Thus a living, active man, constantly exerts mechanical force, gives off heat, evolves carbonic acid and water, and undergoes a loss of substance.

4. Plainly, this state of things could not continue for an unlimited period, or the man would dwindle to nothing. But long before the effects of this gradual diminution of substance become apparent to a bystander, they are felt by the subject of the experiment in the form of the two imperious sensations called hunger and thirst. To still these cravings, to restore the weight of the body to its former amount, to enable it to continue giving out heat, water, and carbonic acid, at the same rate, for an indefinite period, it is absolutely necessary that the body should be supplied with each of three things, and with three only. These are, firstly, fresh air; secondly, drink-consisting of water in som shape or other, however much it may be adulterated; thirdly, food. That compound known to chemists as proteid matter, and which contains carbon, hydrogen, oxygen, and nitrogen, must form a part of this food, if it is to sustain life indefinitely; and fatty, starchy, or saccharine matters ought to be contained in the food, if it is to sustain life conveniently.

5. A certain proportion of the matter taken in as food either cannot be, or at any rate is not, used; and leaves the body, as excrementitious matter, having simply passed through the alimentary canal without undergoing much change, and without ever being incorporated into the actual substance of the body. But, under healthy conditions, and when only so much food as is necessary is taken, no important proportion of either proteid matter, or fat, or starchy or saccharine food, passes out of the body as such. Almost all real food leaves the body in the form either of water, or of carbonic acid, or of a third substance called urea, or of certain saline compounds.

Chemists have determined that these products which are thrown out of the body and are called excretions, contain

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 leave 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.

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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 stationary 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.

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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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FIG. 1.

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 ab; letters as before.

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

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