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The Reader is requested to correct the following


Page 72,

Page 92,
Page 103,

Page 14, line 28, for “contract” read "contact."
Page 34, fig. 10, L. pointing to the left auricle should be L.A.
Page 40, fig. 14, line


of explanation, for "c" read b.Page 63, line 36, for “ Lancelot” read “ Lancelet.” Page 65, line 15, for Kread H.


19, for one-tenth” read one-thirteenth.” Page 78, line 10, for "by carbonic acid ” read “ from carbonic acid.” Page 83, line 12, for 90°” read 989." Page 84, line 4, for clearlyread nearly.”


21, for as ar" readso far." line

31, after food" insert "some constituents of the bile.” Page 122,

line of explanation of fig. 37, for Eread b.” Page 166, line 4, for “risingread “raising." Page 168, fig. 50, line 3 of explanation, after“ between” insert“ these.” Page 186, line 17, formdfread madf." Page 207, fig. 70, line i of explanation, for "tympanitic” read “tym

panic.” Page 215, fig. 71, the left-hand figure should be marked A. Page 227,


22, omit the words "against it.” Page 252, line 36, after of” insert any." Page 263, line 2 from bottom, formedium” read “median.” Page 275, line 8, for “two layers” read“ one layer."








1. The body of a living man performs a great diversity of actions, some of which are quite obvious ; others require more or less careful observation ; and yet others can be detected only by the employment of the most delicate appliances of science.

Thus, some part of the body of a living man is plainly always in motion. Even in sleep, when the limbs, head, and eyelids may be still, the incessant rise and fall of the chest continue to remind us that we are viewing slumber and not death.

More careful observation, however, is needed to detect the motion of the heart ; or the pulsation of the arteries ; or the changes in the size of the pupil of the eye with varying light ; or to ascertain that the air which is breathed out of the body is hotter and damper than the air which is taken in by breathing

And lastly, when we try to ascertain what happens in the eye when that organ is adjusted to different distances :


or what in a nerve when it is excited : or of what materials flesh and blood are made : or in virtue of what mechanism it is that a sudden pain makes one start-we have to call into operation all the methods of inductive and deductive logic; all the resources of physics and chemistry ; and all the delicacies of the art of experiment.

2. The sum of the facts and generalizations at which we arrive by these various modes of inquiry, be they simple or be they refined, concerning the actions of the body and the manner in which those actions are brought about, constitutes the science of Human Physiology. An elementary outline of this science, and of so much anatomy as is incidentally necessary, is the subject of the following Lessons; of which I shall devote the present to an account of so much of the structure and such of the actions (or, as they are technically called, “functions”) of the body, as can be ascertained by easy observation ; or might be so ascertained if the bodies of men were as easily procured, examined, and subjected to experiment, as those of animals.

3. Suppose a chamber with walls of ice, through which a current of pure ice-cold air passes; the walls of the chamber will of course remain unmelted.

Now, having weighed a healthy living man with great care, let him walk up and down the chamber for an hour. In doing this he will obviously exercise a great amount of mechanical force ; as much, at least, as would be required to lift his weight as high and as often as he has raised himself at every step. But, in addition, a certain quantity of the ice will be melted, or converted into water ; showing that the man has given off heat in abundance. Furthermore, if the air which enters the chamber be made to pass through lime-water, it will cause no cloudy white precipitate of carbonate of lime, because the quantity of carbonic acid in ordinary air is so small as to be inappreciable in this way. But if the air which passes out is made to take the same course, the lime-water will soon become milky, from the precipitation of carbonate of lime, showing the presence of carbonic acid, which, like the heat, is given off by the man.

Again, even if the air be quite dry as it enters the chamber (and the chamber be lined with some material so as to shut out all vapour from the melting ice walls), that which

once more.

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. 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 tne 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 some 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 with 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.

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

I 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.)

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