The main disadvantages of vegetable food are equally obvious. It generally contains but a small proportion of the protein com pounds. And even this limited quantity is often virtually diminished by their insoluble state ; or by the indigestible form which is implied by their mechanical arrangement in the vegetable tissues. Alany of its amyla ceous constituents are also rendered useless in the same way : being enclosed in insoluble envelopes, which effectually shield them from the digestive process ; or having a composi tion which requires to be altered by a chemi cal metamorphosis before they can be fitted for absorption. These objections can be to a great extent obviated by the ingestion of a larger quantity of such food, as well as by a more protracted sojourn in the alimentary canal. But, besides these disadvantages, the inorganic constituents of certain kinds of vegetable food appear to be insufficient for the replacement of the loss consequent on the waste of the animal. Thus the ash of many esculent vegetables is peculiarly deficient in the important ingredients of soda and the chlorides. The poisonous materials con tained in the tissues of some plants constitute another objection to vegetable diet ;— an objection which is, however, generally ob viated by the instinct of animals, and by the experience of Alan, or by the purification which the process of cooking often affords.
Animal food. — The muscular substance, accompanied by more or less of its intersti tial and investing fat and areolar tissue, forms what is called meat or flesh, in the ordinary acceptation of' these words.
The mechanical subdivision of a mass of meat would of course afford us the micro scopic elements of the above tissues; —natnely, sareolemma, sarcous substance, white and yellow fibrous elements, fat, and blood vessels ; together with a certain quantity of blood, and of the nutritional fluids which sa turate each of these textures. Its chemical composition varies, not only with the nature, but also with the age, food, habits, and indi vidual peculiarities, of the animal yielding it. Hence it is impossible to give any definite account of its quantitative chemistry. We can only enumerate its principal constituents ; and, in the case of some of the more im portant of them, approximatively estimate their amount. The protein-compound, that forms by far the greater part of the mus cular fibres, is a substance which possesses characters closely allied to those of fibrin, and has received the name of syntonin. It is usually present in a proportion of,about 15 or 16 per cent. The albumen of the juice which soaks the whole muscular mass, and the gelatin which is extracted from it by boiling, may each be estimated at about 2 per cent. Its extractive, exclusive of salts, amounts to about 3 per cent. ; of which nearly half is dissolved by alcohol, half by water. This constituent has a very, complex composition : osmazom, lactic acid, inosit, kreatin, kreatinin, and a variety of other substances, having been detected in it by the labours of modern chemists. The salts of meat form about I+ per cent. of its fresh substance, or about 5 per cent. of its dried mass ; nearly three-fourths of their quantity being phosphates of the alkalies, and two-thirds of the remainder phosphates of the earths, with a little iron. The chlorides of the alkalies are about one-fourteenth of the entire ash. They are remarkably contrasted
with the chlorides contained in the ash of the blood, by the great proportion which the chloride of potassium bears to that of sodium.
It is impossible to estimate the quantity of fat contained in meat as usually eaten. But even after the removal of all visible adipose tissue, Von Bibra has found fractions ranging from one -twentieth to one-fifth ; the smaller amounts corresponding to the flesh of the Hare and Deer, while the larger (in the beef of Oxen) were perhaps partially due to a more or less artificial fattening.
The flesh of Birds contains less water and fat, and rnore albumen, syntonin, and kreatin, than that of most of the Marnmalia hitherto examined. The muscular substance of Fishes contains a still greater quantity of albumen. That of the young of most animals is softer, and its fibres smaller and more digestible, than the flesh of the adult.
The artificial preparation of animal food for the table probably induces a variety of chemical changes. But the full import of these changes has yet to be made out. At pre sent, we know little except some of the more obvious physical results which attend the processes of cooking. These are best seen in the cooking of meat.
The increased digestibility of meat which has been killed some time previously to being eaten, seems to depend, partly on the more uniform and softer consistence imparted by the diffusion of its juices, and partly on the imperfect decomposition which it has begun to undergo. The latter change to some ex tent prepares it for digestion, by rendering it more soluble. But any approach to abso lute putrefaction reverses this advantage ;—at any rate, in the case of Man, whose natural judgment would probably in most instances lead him to reject putrid meat, as alike dis gusting to the senses, hurtful to digestion, and dangerous to health.
In the operation of roasting meat, the heat applied to the exterior of the mass soon con verts its superficial portion into a dense, hard substance. This compact crust consists chiefly of albumen which has been coagulated by heat. It is of essential service, not only in mode rating the heat afterwards applied through it to the deeper portions of the meat, but also in retaining its various liquid and volatile pro ducts, which would otherwise be soon dissi pated in the gaseous or vaporous form. The moderate heat which permeates the mass probably' aids the various juices of the meat in diffusing themselves throughout its whole texture; increasing its uniformity of consist ence, and dissolving much of its gelatinous tissues. Its albumen is always more or less coag.ulated by the heat ; though, where much blood is present, the colour and fluidity which it sometimes retains, appear to indicate an imperfect character of this change.* A variety of empyreumatic substances, which are de veloped chiefly in the more heated exterior of the rnass, next adcl the savoury odour and deepened colour, so characteristic of this method of cooking. If the process be unduly protracted, it will obviously burn the harder outside shell, and render the coagulated and contracted mass within too dense, tough, and insoluble for easy digestion ; while, if con ducted too rapidly, the same combustion of the outside is of course attended with the loss of all the advantages of cooking in the raw central portion.