The Digestive System 139 



large globules of fat into very small droplets, that is, the fat is emulsi- 

 fied, making available a greater surface for the action of the enzymes. 

 This is strictly a physical change. 



The pancreatic juice contains three enzymes. These enzymes are : 

 amylase which acts upon any starches that escaped the ptyalin, con- 

 verting them into double sugars (disaccharides) ; trypsin which continues 

 the work started by the pepsin of the stomach and converts the partially 

 digested proteins into amino acids ; and the lipase which converts fats into 

 fatty acids and glycerols. 



The intestinal juice likewise has several enzymes. One of these, 

 ercpsin, has the same general action as trypsin, that is, it completes the 

 digestion of the proteins. The other three enzymes are concerned 

 with changing the double sugars into simple sugars (monosaccharides) : 

 maltase acts on maltose (malt sugar), lactase acts on lactose (milk 

 sugar), and invertase (or sucrase) on sucrose (cane sugar). Another 

 component of intestinal juice is enter okinase which activates the trypsin 

 of the pancreatic juice. Trypsin is actually secreted as trypsinogen and 

 is changed by the enterokinase into trypsin. 



There are no digestive enzymes in the large intestine of most mam- 

 mals. Among those herbivores with a large caecum, food may be re- 

 tained for some time as digestion continues. Undigested portions of 

 food are collected in the large intestine, and excess water is absorbed. 

 This results in changing the semiliquid mass into the more solid jeces 

 which are then eliminated. The numerous glands of the large intes- 

 tines secrete a lubricating material for keeping the feces soft after 

 the water is absorbed. The characteristic color of the feces is due 

 to contained bile pigments. 



Control of the Digestive Glands. — One intriguing aspect of the 

 digestive process is the manner in which the glands are activated to 

 secrete. It is a familiar fact that saliva will start flowing copiously at 

 the mere mention of some particularly savory food. Odors will bring 

 about the same reaction ; also the presence of food in the mouth ap- 

 pears to increase the salivary flow. It appears, then, that the flow of 

 saliva is under the control of nerves. 



The presence of food in the stomach is, in part, sufificient to start 

 the secretion by the gastric glands. In fact, it has been shown that 

 the presence of food in the mouth or even the odor of food will like- 

 wise initiate this flow. This appears to demonstrate that this flow 

 is at least to a great extent under nervous control. It has been shown, 



