THE ANIMAL BODY— DIGESTION— METABOLISM 25 



Dig'estion in the small intestine. — When received into the small 

 intestine, the partially digested food is a semi-liquid mass. As yet, 

 the fats in the food have not been digested, and the digestion of the 

 proteins and carboh3'drates is far from complete. Here the work of 

 digestion proceeds even more vigorously than in the stomach, all classes 

 of nutrients being attacked. The small intestine receives near its 

 upper part digestive fluids from two outside organs, the liver and the 

 pancreas, and another digestive juice is secreted in the wall of the 

 intestine itself. Immediately on entering the small intestine the 

 inpouring material is changed from an acid to an alkaline character 

 thru the rapid addition of bile and pancreatic juice, both alkaline. 

 This stops the action of the pepsin, which works only when acid is 

 present. 



The pancreatic juice. — The pancreatic juice is produced by the 

 pancreas, or sweetbread, a slender gland lying just beyond the stomach 

 and connected with the small intestine by a duct. The chief enzymes 

 it contains are trj-psin, amylase, and lipase. Trypsin, like pepsin, 

 changes protein into proteoses and peptones, and is also able to break 

 some of these partially digested substances further into amino acids. 

 It is believed that before the food protein can be absorbed and used by 

 the animal body it must all be cleaved into amino acids, which, as we 

 have seen in Chapter I, are the simple "building stones" from which 

 proteins are formed. Amylase changes starch into sugar. Lipase 

 splits fats into fatty acids and glycerin. The fatty acids unite with 

 alkalies in the bile to form soaps, and are absorbed from the intestine 

 in this form. 



The bile. — The bile, secreted by the liver, the largest organ in the 

 body, is a greenish or golden colored fluid, alkaline and extremely 

 bitter in taste. It contains no enzymes but is nevertheless exceedingly 

 important in digestion, as it furnishes the alkalies necessary to change 

 the fatty acids formed by lipase into soaps. It also aids in emulsifying 

 the undigested fat; i.e., breaking it up into very minute droplets, so 

 that it can be more readily acted on by the lipase. Furthermore, in 

 some manner the bile increases the digestive power of the pancreatic 

 and intestinal juices. After performing its work, much of the bile is 

 absorbed from the intestine and, passing back to the liver, is used once 

 more. 



The intestinal secretion. — The digestive fluid secreted by the mucous 

 membrane of the small intestine contains several enzymes, the most 

 important of which are erepsin and the invertases. Erepsin attacks 

 the proteoses and peptones which have escaped the action of trypsin 

 and breaks them up into amino acids. It can not act on protein which 

 has not already been split into proteoses and peptones. The invertases 



