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; f, 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 gf, 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.

The velum with its uvula comes into contact below with the upper part of the back of the tongue, and with a sort of gristly, lid-like process connected with its base, the epiglottis (e).

Behind the partition thus formed lies the cavity of the pharynx, which may be described as a funnel-shaped bag with muscular walls, the upper margins of the slanting, wide end of which are attached to the base of the skull, while the lateral margins are continuous with the sides, and the lower with the floor, of the mouth. The narrow end of the pharyngeal bag passes into the gullet or œsophagus (b), a muscular tube, which affords a passage into the stomach.

There are no fewer than six distinct openings into the front part of the pharynx-four in pairs, and two single ones in the middle line. The two pairs are, in front, the hinder openings of the nasal cavities; and at the sides, close to these, the apertures of the Eustachian tubes (g). The two single apertures are, the hinder opening of the mouth between the soft palate and the epiglottis ; and, behind the epiglottis, the upper aperture of the respiratory passage, or the glottis.

14. The mucous membrane which lines the mouth and the pharynx is beset with minute glands, the buccal glands; but the great glands from which the cavity of the mouth receives its chief secretion are the three pairs which, as has been already mentioned, are called parotid, submaxillary, sublingual, and which secrete the principal part of the saliva (Fig. 41).

Each parotid gland is placed just in front of the ear, and its duct passes forwards along the cheek, until it opens in the interior of the mouth, opposite the second upper grinding tooth.

The submaxillary and sublingual glands lie between the lower jaw and the floor of the mouth, the submaxillary being situated further back than the sublingual. Their ducts open in the floor of the mouth below the tip of the tongue. The secretion of these salivary glands, mixed with that of the small glands of the mouth, constitutes the saliva—a fluid which, though thin and watery, contains a small quantity of animal matter, called Ptyalin, which has certain very peculiar properties. It does not

act upon proteid food-stuffs, nor upon fats; but if mixed with starch, and kept at a moderate warm temperature, it turns that starch into grape sugar. The importance of this operation becomes apparent when one reflects that

A dissection of the right side of the face, showing a, the sublingual; b, the submaxillary glands, with their ducts opening beside the tongue in the floor of the mouth at d; c, the parotid gland and its duct, which opens on the side of the cheek at e.

starch is insoluble, and therefore, as such, useless as nutriment, while sugar is highly soluble, and readily oxidizable..

15. Each of the thirty-two teeth which have been mentioned consists of a crown which projects above the gum, and of one or more fangs, which are embedded in sockets, or what are called alveoli, in the jaws.

The eight teeth on opposite sides of the same jaw are constructed upon exactly similar patterns, while the eight teeth which are opposite to one another, and bite against one another above and below, though similar in kind, differ somewhat in the details of their patterns.

The two teeth in each eight which are nearest the middle line in the front of the jaw, have wide but sharp and chisel-like edges. Hence they are called incisors,