The absorption of fatty matter is, however, materially influenced by the bile. So that, when this secretion is altogether diverted from its accustomed channel, its absence from the intestinal canal reduces the quantity of fat taken up here to about one-fourth of that normally absorbed.
The quantity of biliary solids discharged with the fleces is but a very small fraction of that poured into the bowel ;—probably not more than -1'„,th or Ath. And much of this may be regarded as a precipitate ; which is produced, partly by an oxidation of a small part of the biliary fats into resinous matter, and partly by the combination of the fatty acids of the bile with the lime taken in the food.
The details of the assistance given by the bile to digestion, are very obscure. But as suming the accuracy of M. Bernard's views with respect to the uancreatic function, we may conjecture, that it is the venous, rather than the lacteal absorption of fat, which is furthered by the hepatic secretion. This view agrees with the known capacity of the soda in the bile to unite with fatty acids in the form of a soap. And that separation of the neutral fats of the food into acid and base, which would probably precede such a sapo nification, is sufficiently explained by their sojourn in the intestine at the ordinary tem perature; or by the direct effect of this kind which the pancreatic fluid could produce, supposing it not overpowered by the gastric juice. But Bidder and Schmidt offer the fur ther suggestion, that even neutral fat is aided in penetrating the villus by the moistening of its surface with bile :—a view which they confirm from experiments with capillary tubes and dead animal membranes.
The continuous absorption of these various alimentary principles constitutes the chief share taken by the remainder of the intestinal canal in the function of digestion. And as this absorption requires contact, too rapid a transit of the canal, or too limited an extent of digestive tube to be traversed, present much the same appearances in the food, and produce a very similar effect on the organism. Thus the iMces expelled in intestinal fistula, on the one hand, or in diarrhcea on the other, alike exhibit a large quantity of undigested starch, protein, and fat ; with unabsorbed bile and water. And the fatal exhaustion which often accompanies botli of these states, is evidence of the virtual starvation which may thus be brought about.
From the little change undergone by pro teinous substances artificially introduced into the large intestine, as well as from the com parative development of this part of the canal in the carnivorous and herbivorous classes, We may conjecture that its absorptive powers are chiefly intended to be exercised on the vvater of its contents; and on the sugar and lactic acid produced by that slow metamorphosis, which dense starchy substances would here continue to undergo. But in animals like the Horse, whose aliment passes quickly through the stomach and small intestine into an enormous colon, it is difficult to avoid believ ing, that a more or less modified gastric juice accompanies the insoluble albuminous com pounds of the food into this segment of the canal, and continues its solvent action during their long sojourn in its interior. It would otherwise be almost inipossible to explain the nutrition of such animals. How far the large intestine can take up fat remains un known. But it seems certain that its share in the absorption of this alimentary principle is very slight compared with that of the small intestine.
The entire process of digestion might there fore be described as consisting in the applica tion to the food of a variety of agencies, such as mechanical division, solution, and metamor phosis. In whatever manner these are ap plied (either to the tbod as a whole, or to the several alimentary principles which form its constituents), and whether they operate in suc cession or combination—in any case, they all work towards the same object : namely, that of preparing the food for absorption by the vessels and lacteals which occupy the walls of the digestive canal. With this act of ab sorption, the function of digestion terminates.
The chief agents of this process of division and solution, we have found to consist of certain liquid secretions ; which are poured into the canal, either bk the ducts of several glands, or by the vast compound mucous mem brane that lines the various parts of its cavity. In short, the food received into the intestinal tube, mingles with a large quantity of a mixed fluid ; which itself represents the aggregate contributions of the salivary glands, the pancreas, the liver, the stomach, and the in testine.