Now, when it is recollected, that the elements of these alimentary substances are nearly the same, and that from the varied proportions and different combinations of these elements is produced the almost countless variety of vegetable and animal matters, we are enabled to understand, in a general way, how aliments so diver sified in structure and sensible qualities become assimi lated to our own system.
The food, being previously masticated and combined in the mouth with saliva and air, is received into the stomach, where it is exposed to the action of the gas tric fluid, a powerful solvent of animal and vegetable matters. Here it is soon reduced to the state of a soft pultaceous mass, having suffered a peculiar solution, de composition, and new arrangement of its constituent parts, which may be called digestive ; and which, so far from being similar, is always, in the healthy condi tion of the stomach, opposed to those spontaneous changes which terminate in the acetous and putrefactive fermentations. From the stomach the digested chyme passes into the intestines ; where, subjected to the ac tion of the bile, the pancreatic and mucous secretions, it undergoes still farther changes ; the result of all which is the formation and separation of a bland white milky fluid, the chyle. The chyle is sucked up by numerous vessels, called absorbent lacteals, to whose orifices it is ever) where exposed in passing through the intestinal canal. These absorbents, after numerous communica tions, terminate in one common trunk, by which the chyle is at length carried into the blood near the heart. Thus the chyle is mixed with the blood, and subjected to the action of the heart and arteries. Circulated now through the lungs, it undergoes new changes from the respira tion of the atmosphere ;—it is incorporated with the common circulating mass, and becomes itself blood, the fountain from which all the other constituent parts of the body are formed and renewed.
The different processes of digestion, chylification, and assimilation, seem nothing more indeed than particular modes of decomposition and recombination of the con stituent elements of alimentary matters. Even animal matters, though containing all the proximate principles of our bodies, already formed, must, when assumed as aliments, undergo in these different processes the same changes as vegetable food. They must in like manner
be dissolved, digested, decomposed, ana again combined. that chyle may be formed ; from which, in the process of assimilation and nutrition are produced the different animal principles.
Alimentary substances, then, promote the growth, support the strength, and renew the waste of the sys tem, in proportion to their digestibility, and to the quan tity of chyle they are capable of affording. The gas tric fluid of man is capable of digesting a great variety of animal and vegetable matters. And the structure of his body, his instincts, and experience; clearly spew, that he has been destined to derive his aliment from both kingdoms of nature.
Of these, animal food is the more nutritious ; but it seems at the same time, from its putrescent and stimu lating nature, not to be suited to form the whole of our daily aliment. And in fact, if long and exclusively used, animal food overheats and stimulates, and at length ex hausts and debilitates the system which it had at first invigorated and supported. Those accordingly who have lived for any great length of time on a diet com posed entirely of animal matters, become oppressed, heavy, and indolent ; the tone and excitability of their frame are impaired ; they are afflicted with indigestion; the breathing is hurried on the smallest exercise ; the gums swell and bleed ; the breath is foetid, and the limbs are inactive, stiff, and swollen.
We recognise in this description, the approach of scurvy, a disease familiar to sailors, to the inhabitants of besieged towns, and in general to all who are wholly deprived of a just proportion of fresh vegetable aliment.
On the other hand, vegetables are acescent, and less stimulating ; they are also less nourishing, and of more difficult assimilation than food derived from the animal kingdom. Hence it is, perhaps, that nature has provid ed a greater extent of digestive organs for animals whol ly herbivorous. A diet, however, entirely vegetable, seems insufficient to raise the human system to all the strength and vigour of which it is susceptible. Flatu lence and acidity of the stomach, muscular and nervous debility, and a long train of hysterical and hypochon driacal disorders, are not unfrequently the consequence of this too sparing diet.