nitrogen (at any rate, in any considerable quantity) can be taken into the blood, and that is in the form of a solution of proteid matter. If proteid matter be not supplied, therefore, the body must needs waste, because there is nothing in the food competent to make good the loss of nitrogen.

On the other hand, if proteid matter be supplied, there can be no absolute necessity for any other but the mineral food-stuffs, because proteid matter contains carbon and hydrogen in abundance, and hence is competent to give origin to the other great products of waste, carbonic acid and water.

In fact, the final results of the oxidation of proteid matters are carbonic acid, water, and ammonia; and these, as we have seen, are the final shapes of the waste products of the human economy.

7. From what has been said, it becomes readily intelligible that, whether an animal be herbivorous or carnivorous, it begins to starve from the moment its vital food-stuffs consist of pure amyloids, or fats, or any mixture of them. It suffers from what may be called nitrogen starvation, and, sooner or later, will die.

In this case, and still more in that of an animal deprived of vital food altogether, the organism, so long as it continues to live, feeds upon itself. In the former case, those excretions which contain nitrogen, in the latter, all its waste products, are necessarily formed at the expense of its own body; whence it has been rightly enough observed that a starving sheep is as much a carnivore as a lion.

8. But though proteid matter is the essential element of food, and under certain circumstances may suffice, by itself, to maintain the body, it is a very disadvantageous and uneconomical food.

Albumen, which may be taken as the type of the proteids, contains about 53 parts of carbon and 15 of nitrogen in 100 parts. If a man were to be fed on white of egg, therefore, he would take in, speaking roughly, 3 parts of carbon for every part of nitrogen.

But it is proved experimentally, that a healthy, fullgrown man, keeping up his weight and heat, and taking a fair amount of exercise, eliminates 4,000 grains of

carbon to only 300 grains of nitrogen, or, roughly, only needs one-thirteenth as much nitrogen as carbon. However, if he is to get his 4,000 grains of carbon out of albumen, he must eat 7,547 grains of that substance. But 7,547 grains of albumen contain 1,132 grains of nitrogen, or nearly four times as much as he wants.

To put the case in another way, it takes about four pounds of fatless meat (which generally contains about one-fourth its weight of dry solid proteids) to yield 4,000 grains of carbon, whereas one pound will furnish 300 grains of nitrogen.

Thus a man confined to a purely proteid diet, must eat a prodigious quantity of it. This not only involves a great amount of physiological labour in comminuting the food, and a great expenditure of power and time in dissolving and absorbing it; but throws a great quantity of wholly profitless labour upon those excretory organs, which have to get rid of the nitrogenous matter, three-fourths of which, as we have seen, is superfluous.

Unproductive labour is as much to be avoided in physiological, as in political, economy; and it is quite possible that an animal fed with perfectly nutritious, proteid matter should die of starvation: the loss of power in various operations required for its assimilation overbalancing the gain; or the time occupied in their performance being too great to check waste with sufficient rapidity. The body, under these circumstances, falls into the condition of a merchant who has abundant assets, but who cannot get in his debts in time to meet his creditors.

9. These considerations lead us to the physiological justification of the universal practice of mankind in adopting a mixed diet, in which proteids are mixed either with fats, or with amyloids, or with both.

Fats may be taken to contain about 80 per cent. of carbon, and amyloids about 40 per cent. Now it has been seen that there is enough nitrogen to supply the waste of that substance per diem, in a healthy man, in a pound of fatless meat; which also contains 1,000 grains of carbon, leaving a deficit of 3,000 grains of carbon. Rather more than half a pound of fat, or a pound of sugar, will supply this quantity of carbon. The former, if properly subdivided, the latter, by reason of its

solubility, passes with great ease into the economy, the digestive labour of which is consequently reduced to a minimum.

10. Several apparently simple articles of food constitute a mixed diet in themselves. Thus butcher's meat commonly contains from 30 to 50 per cent. of fat. Bread, on the other hand, contains the proteid, gluten, and the amyloids, starch and sugar, with minute quantities of fat. But, from the proportion in which these proteid and other constituents exist in these substances, they are neither, taken alone, such physiologically economical foods as they are when combined in the proportion of about 200 to 75; or two pounds of bread to three-quarters of a pound of meat per diem.

11. It is quite certain that nine-tenths of the dry, solid food which is taken into the body sooner or later leaves it in the shape of carbonic acid, water, and urea (or uric acid); and it is also certain that the compounds which leave the body not only are more highly oxidized than those which enter it, but in them is carried away out of the body all the oxygen taken into the blood by the lungs.

The intermediate stages of this conversion are, however, by no means so clear. It is highly probable that the amyloids and fats are very frequently oxidized in the blood, without, properly speaking, ever forming an integral part of the substance of the body; but whether the proteids may undergo the same changes in the blood, or whether it is necessary for them first to be incorporated with the living tissue, is not positively known.

So, again, it is certain that, in becoming oxidized, the elements of the food must give off heat, and it is probable that this heat is sufficient to account for all that is given off by the body; but it is possible, and indeed [probable, that there may be other minor sources of heat.

12. Food-stuffs have been divided into heat-producers and tissue-formers-the amyloids and fats constituting the former division, the proteids the latter. But this is a very misleading classification, inasmuch as it implies, on the one hand, that the oxidation of the proteids does not develop heat; and, on the other, that the amyloids and fats, as they oxidize, subserve only the production of heat.

Proteids are tissue-formers, inasmuch as no tissue can be produced without them; but they are also heatproducers, not only directly, but because, as we have seen (Lesson V. §§ 25, 26), that they are competent to give rise to amyloids by chemical metamorphosis within the body.

If it is worth while to make a special classification of the vital food-stuffs at all, it appears desirable to distinguish the essential food-stuffs, or proteids, from the accessory food-stuffs, or fats and amyloids-the former alone being, in the nature of things, necessary to life, while the latter, however important, are not absolutely

necessary.

13. All food-stuffs being thus proteids, fats, amyloids, or mineral matters, pure or mixed up with other substances, the whole purpose of the alimentary apparatus is to separate these proteid, &c. from the innutritious residue, if there be any; and to reduce them into a condition either of solution or of excessively fine subdivision, in order that they may make their way through the delicate structures which form the walls of the vessels of the alimentary canal. To these ends food is taken into the mouth and masticated, is mixed with saliva, is swallowed, undergoes gastric digestion, passes into the intestine, and is subjected to the action of the secretions of the glands attached to that viscus; and, finally, after the more or less complete extraction of the nutritive constituents, the residue, mixed up with certain secretions of the intestines, leaves the body as the faces.

The cavity of the mouth is a chamber with a fixed roof, formed by the hard palate (Fig. 40, 7), and with a moveable floor, constituted by the lower jaw, and the tongue (k), which fills up the space between the two branches of the jaw. Arching round the margins of the upper and the lower jaws are the thirty-two teeth, sixteen above and sixteen below, and, external to these, the closure of the cavity of the mouth is completed by the cheeks at the sides, and, by the lips, in front.

When the mouth is shut, the back of the tongue comes into close contact with the palate; and, where the hard palate ends, the communication between the mouth and the back of the throat is still further impeded by a sort of

fleshy curtain--the soft palate or velum-the middle of which is produced into a prolongation, the uvula (f), while its sides, skirting the sides of the passage, or fauces,

A SECTION OF THE MOUTH AND NOSE TAKEN VERTICALLY, A LITTLE TO THE LEFT OF THE MIDDLE LINE.

a, the vertebral column; b, the gullet; c, the windpipe; d, the thyroid cartilage of the larynx; e, the epiglottis; the uvula; g, the opening of the left Eustachian tube; h, the opening of the left lachrymal duct; i, the hyoid bone; k, the tongue;, the hard palate; m, n, the base of the skull; o, p, q, the superior, middle, and inferior turbinal bones. The letters Sf, e are placed in the pharynx.

form double muscular pillars, which are termed the pillars of the fauces. Between these the tonsils are

situated, one on each side.