246 EEALTH AND DISEASE 



changes, so that from being acid the chyme becomes alkaline. The bile 

 not only neutralizes the acidity of the latter, but it aids in the emulsi- 

 fication and saponification of the oils and fats. It also promotes their 

 absorption, and it acts as a stimulant to the muscular walls of the intestine, 

 causing them to contract, and assisting in this way the onward movement 

 of the intestinal contents. It seems to interfere with or arrest the further 

 action of the gastric juice on the proteids. That action, however, is recom- 

 menced by the pancreatic juice, which can form soluble peptones from the 

 insoluble proteids in an alkaline medium, just as the gastric juice did in an 

 acid one. The pancreatic juice is in fact the most important of the digestive 

 fluids. Comparatively small in quantity, coagulating into a solid mass by 

 heat, it is found to contain three ferments, named trypsin, steapsin, and 

 amylopsin. Trypsin converts proteids into soluble peptones, but in a 

 somewhat different way from that in which the change is effected by gastric 

 juice, and it can carry the change one step further by decomposing them 

 into leucin and tyrosin. Steapsin possesses the power first of emulsionizing 

 and then of saponifying the oleaginous compounds, thus aiding in their 

 absorption ; and lastly — the amylopsin effects similar changes in the starches 

 to those which we have just seen are accomplished by the saliva. 



The intestinal juice appears to have an action resembling, though less 

 powerful than, that of the pancreatic secretion, acting upon all the organic 

 constituents of the food, rendering them soluble and aiding in their 

 absorption. 



From the small intestine the now greatly modified food mass passes 

 into the large intestine; and when the enormous capacity of the csecum 

 and colon are considered, it is difficult to avoid the conclusion that a still 

 further effort is made to convert the materials that have hitherto success- 

 fully resisted the warmth, moisture, and powerful action of the several 

 digestive fluids secreted by the salivary glands, the stomach, liver, pancreas, 

 and intestinal mucous membrane, into useful nutritive substances, and at 

 the same time to effect their absorption. A considerable development of 

 infusory animalcules, as well as of micro-organisms in the form of bacilli 

 and micrococci, takes place in the large intestine. This subject may be 

 concluded by giving the results of the observations of Colin in regard to 

 the progress of the food through the alimentary canal. He kept the 

 animals fasting for twenty-four hours from solids and liquid, then fed them 

 with about 5\ lbs. of hay, and about 2\ gallons of water, to which must 

 be added an estimated equal quantity of saliva, and killed them at different 

 periods. Two hours after the beginning of the meal the stomach had passed 

 into the intestine one-half in weight of all that had been ingested; two- 

 fifths of the dry hay had been retained, the remaining three-fifths had 



