ENZYMES. FERMENTATION 397 



Digestion in Animals. 



The three classes of organic compounds, the fats, the car- 

 bohydrates and the proteins, are taken in as food and are hydrolysed 

 into their constituents before they can pass through the wall of the 

 alimentary canal and can be assimilated. 



I. Saliva. 



The first digestion of food occurs in the mouth by the action of 

 the saliva, the secretion of the salivary glands. The saliva con- 

 tains the enzyme diastase, or amylase, which hydrolyses starch (and 

 glycogen) converting it into dextrin and maltose. In the mouth, 

 however, very little enzyme action takes place, the food being only 

 moistened by the saliva and swallowed. The action occurs in the 

 stomach, where the food is in the form of a mass in the fundus ; here 

 only the exterior of the mass is in contact with the hydrochloric acid 

 of the gastric juice, which inhibits the action of the diastase ; the starch 

 in the interior of the mass is slowly digested. 



II. Gastric Juice. 



The first hydrolysis of proteins occurs in the stomach by the 

 enzyme pepsin. Pepsin is secreted by certain cells of the gastric 

 mucous membrane. It acts only in the presence of hydrochloric 

 acid, which is secreted by other cells. In disease, lactic acid is 

 sometimes found in the contents of the stomach. Besides pepsin, 

 the enzyme, rennin, is present in the cells of the mucous membrane, 

 but it is very probable that pepsin and rennin are identical. Rennin 

 acts upon caseinogen, the protein of milk, converting it into casein 

 (see under milk, p. 461). 



III. Pancreatic Juice. 



From the stomach the food passes into the intestine. The acid 

 contents of the stomach, when they pass into the duodenum and 

 come in contact with its mucous membrane, induce the secretion of 

 secretin. The secretin passes into the blood and is carried to the 

 pancreas, where it excites a flow of pancreatic juice. This juice has 

 very little action upon proteins, but it contains lipase and diastase 

 which hydrolyse fats and starch respectively. As soon as the pancre- 

 atic juice, which contains trypsinogen, comes into the intestine, it 

 becomes activated by the enzyme enterokinase and converted into the 

 powerful proteoclastic enzyme, trypsin. Enterokinase is secreted by 

 the glands of the duodenum. Trypsin acts upon unchanged proteins, 

 proteoses, etc., from the stomach and converts them almost entirely 

 into amino acids. A complex polypeptide is also formed which is not 

 acted upon by trypsin. 



