The starch of the chyme would seem to be converted into sugar, hy the addition of a further quantity of an agent, similar to that which is furnished by the mixed secretions of the salivary glands and mouth. At any rate, the secretion of the pancreas, which is poured out into the 'duodenum with the bile, is gifted with the capacity of inducing this changejust as rapidly as the mixed saliva itself.
The fatty constituents of the food are probably absorbed by two channels, if not by two processes. But the quantitative share taken by each of these, remains at present unknown. That a certain portion of the fat contained in the food is taken up by the vessels of the alimentary canal, seems evident from the remarkable difference in the amount of this substance, which is found in the organic residuum of the portal blood, and that of the ordinary systemic veins.t But the quantity thus absorbed can scarcely be large. It ap pears to consist chiefly of the more fluid elain. And hence there seem no valid grounds for the supposition of much assistance being given to its transudation from the digestive canal into the veins, by means of a saponi fication with their alkaline blood. A much greater quantity of the fatty matter of the food is taken up by the lacteals of the villi, and is conveyed from these vessels into the thoracic duct. The microscopic details of this process have already been mentioned. Hence, it only remains for us to notice its chemical relations to the various secretions poured into the small intestine, in the lacteals of which segment of the canal the white or fatty chyle is chiefly found.
The experiments and observations of Claude Bernard would ascribe the formation of chyle chiefly to the pancreatic juice. This secretion appears to have the power of se parating fats into their acid and base. But the fact, that such a saponification is pre vented by an admixture of gastric juice, or any other acid, would justify us in doubting whether the change really occurs in the acid chyme of the living body. And all the appearances of the chyle in the lacteals of the villi concur in representing their fatty contents as being not saponified, but merely in a state of minute division. The production of this con dition,—which closely corresponds to that of oily substances when reduced to an artificial einulsion,—would seem to be one of the chief offices of the pancreatic juice.
The evidence adduced by Bernard* in sup port of his views appears very conclusive. His experiments gave hien the means of ob taining large quantities of pancreatic juice from Dogs. A mixture of this secretion and oil, when shaken together, immediately pro duced an emulsion of the most intimate kind, such as no other animal fluid which he exa mined could imitate. A similar emulsion within the body could be seen in the Rabbit, when fed on butter. In this animal, the pan creatic duct opens into the intestine com paratively- low down. And hence there is, under such circumstances, a long extent of bowel above the orifice of this duct, quite devoid of white chyle ; while below, the lac teals are distended with this fluid. And, finally, experiments on the healthy animalt, and observations on human disease, tend to establish the same conclusion. Complete artificial obstruction of the pancreatic duct, diversion of its contents from the intestine, and extensive disease of the secretory struc ture of the gland, —alike prevent the forma tion of white chyle.
The small quantity of the pancreatic se cretion would, perhaps, indicate that much of the metamorphosis of starch is effected by the saliva. And, taken in conjunction with the neutral or feebly alkaline reaction of the bile, and the apparently small amount of alka line intestinal juice, it may at any rate be re garded as throwing great doubt upon the old theory of a direct neutralization of the acid chyme in the intestinal cavity.
The precise share taken by the bile in the process of intestinal digestion, is even more obscure than that of the pancreatic fluid. It may probably be stated as follows :— The bile is not essential to the solution or absorption of any one of the alimentary prin ciples. Nor, on the other hand, does its pre sence check the conversion of protein into peptone or of starch into sugar. But from the appearance of the fmces in jaundice and biliary fistulx, it would seem that its admix ture with the food limits and modifies the putrefaction of its animal constituent, and the acid fermentation of its vegetable por tions; and thus far aids in its proper assimi lation. While the constipation generally ob served in such cases, indicates that the bile is also a stimulus to the muscular action of the bowels.