The experiments of Dr. Prout agreed gene rally with those of Vauquelin and Marcet; he found the coagulum and the fluid part analogous to the two components of the blood, and he likewise observed the oily matter. He compared the chyle derived from animal, with that from vegetable food, and detected the oil in both of them, and, upon the whole, he found them to differ less than was supposed by Mareet; he remarks, however, that the latter contains more water and less albuminous matter than the former.t We were likewise indebted to Dr. Prout for an interesting ac count of the successive changes which the chyle experiences, from its entrance into the lacteals, until it is finally deposited in the thoracic duct, its gradual conversion into blood corresponding to the progress along the vessels.f While the alimentary mass passes through the small intestines, the chyle, as it is separated from it, is taken up by the lacteals, so that when it arrives at the large intestines, nothing remains but the residuary matter, which is to be discharged from the system ; this consti tutes what has been termed the process of dcfiecation. There can be no doubt that the principal and primary use of the large in testines is to serve as a depository for this residuary mass, yet there are certain circum stances in their anatomical and physiological structure, which might render it probable that some farther purpose is served by them than the mere retention of the faces. Dr. Prout, who has minutely examined the successive changes which the contents of the intestinal canal experience, observes that the secretions even of the rectum still possess the property of coagulating milk, which we noticed above as being one of the most distinguishing cha racters of the digestive system, so that it would seem that these organs, in some way or other, still assist in the process of nutrition. We may presume, however, that this is only a secondary object, and that the primary use of the large intestines is to serve as a reservoir, in which the fiscal mass might be retained, in order to be evacuated at certain intervals only.* (See INTESTINAL CANAL.) Before we dismiss this part of our subject, it may be proper to make a few remarks upon two of the abdominal viscera, which, from their anatomical position and their physiological rela tions, are generally classed among the chylopoi etic organs, as being supposed to contribute to the function of digestion ; these are the pancreas and the spleen. The pancreas bears a very near resemblance to the salivary glands of the mouth and fauces, both from its intimate structure and from the nature of its secretions, and it has been presumed, that it acts in the same manner upon the aliment ;t it must, however, be admitted that we have little but analogy or conjecture in favour of this opinion.
The spleen is an organ whieh, both from its size, its situation, and the number of blood vessels belonging to it, has been supposed to serve some important purpose in the animal economy, and from its apparent connexion with the stomach to be, in some way, concerned in the process of digestion. But although many hypotheses and conjectures have been formed on the subject, there is none which seems to have obtained any credit with physiologists, or indeed to be entitled to much consideration" The latest researches on the subject are those of Home, and of Tiedemann and Gmelin. Home examined the structure of the spleen, and, as the result of his investigation, informs us that it consists entirely of a congeries of bloodvessels and absorbents, and that there are interstices between the vessels into which the blood is effused, through certain natural orifices in the veins, when they are much distended. The conclusion which he forms respecting the use of the spleen is, that it is a reservoir for any super fluous matter, which may exist in the stomach, after the process of digestion is completed, which is not carried off by the intestines, as serum, lymph, globules, and mucus ; that these are conveyed to the spleen by certain communi cating vessels, and are removed from it, partly by the veins and partly by the absorbents.t The account of the structure of the spleen which is given us by Tiedemann and Gmelin is considerably different from that of Home.
They inform us that it essentially resembles that of the lymphatic glands, and they conceive that it is to be regarded as an appendage to the lymphatic system. They suppose its specific function to be the secretion of a fluid which is conveyed to the thoracic duct, and being united with the chyle, converts it into blood./ There are many circumstances which render it pro bable that the spleen, in some way or other, promotes sanguification, and we have some reason to believe, that there is an immediate and a ready communication between its arterial and its absorbent systems, but we conceive that the hypothesis must still be regarded rather as a plausible conjecture, than as a deduction from facts.
There is moreover a circumstance which must not be overlooked in our speculations respecting the spleen, that we have some well authenticated cases, where it has been either originally wanting, or has been removed from the body without apparent injury.§ This argu ment cannot, however, be considered as decisive, because it is well known, that in consequence of the extraordinary compensating powers of the system, certain organs may be occasionally dis pensed with, which, under ordinary circumstan ces, appear the most essential to its existence and welfare. We may therefore conclude with respect to the pancreas and the spleen, that although there is reason to suppose that they contribute, in some way, to the function of di gestion, we are still unable to ascertain the pre cise mode in which they conduce to this end. Before we dismiss this part of our subject, it will be necessary to. make a few observations upon a question, which has been proposed in relation to the digestive process, whether any part of the aliment passes through the stomach, and is taken up by the absorbents, without de composition. It is obvious that this cannot be the case with vegetable substances of any des cription, and with respect to substances of ani mal origin, that form a part of the diet, although they approach so much nearer to the nature of chyle, yet it appears that they are not entirely identical with it, and that they must conse quently be decomposed and assimilated to the general mass, before they can serve for the pur poses of nutrition. There are indeed certain substances, that are received into the stomach, which would appear to form exceptions to this general principle ; these are the various saline substances, which are found in all organized bodies, as well as some others, which give their appropriate odours and flavours to the food, and also certain medical agents. There are some salts, which appear to constitute an essential part of the blood and other animal fluids, and as the same salts are introduced into the sto mach with the food, we may conceive that they pass unchanged into the vessels. There are likewise certain substances which give their specific odour to the milk, and to other secre tions and excretions, proving that they likewise pass into the circulating system without suffer ing decomposition, and the same is the case with some of the medicaments.° IV. Theory of digestion.—We now enter upon the fourth branch of our inquiry, the mode in which we are to explain the action of the di gestive organs upon the aliment. This has been one of the most fertile sources of conjecture and speculation from the earliest period, from Hip pocrates down to our own times, and the ques tion is one respecting which the greatest differ ence of opinion still exists among the most intelligent physiologists.t We shall not think it necessary to notice the opinions of the older writers, which were necessarily formed from very insufficient data, but shall select those hy potheses which appear deserving of more par ticular attention, either as having been supported by men of acknowledged eminence, or as possessing in themselves the merit of consis tency and probability. Those which we shall select are the theories of trituration, of fermen tation, of chemical solution, and of nervous action, under one or other of which we may comprehend all the most important speculations which have engaged the attention of modern physiologists.