1 86 ORGANIC AGRICULTURAL CHEMISTRY 



goes to show that the absorption may be direct. If not, the 

 resynthesis of the fat from the hydrolytic products must be 

 immediate and must yield the same original fat as is fed in the 

 food. The fact should be mentioned that lipolytic enzymes 

 which bring about the hydrolysis of fats may also assist in their 

 resynthesis, as it has been shown that these enzymes primarily 

 possess the power of reversibility, i.e. of producing the reverse 

 reaction. 



RESUME OF DIGESTION AND ABSORPTION 



Before taking up the study of metabolism let us review in 

 outline the course of food through the digestive tract and its 

 absorption into the circulatory system, i.e. the blood. 



Mouth. — In the mouth ptyalin present in the saliva hydro- 

 lyzes starch and dextrin to maltose and some maltose may be 

 further hydrolyzed to glucose by maltase also present. The 

 food then passes down the esophagus to the 



Stomach, where it remains for a short period (one half to 

 two hours) still alkaline from the action of saliva and where 

 ptyalin still continues to work. Eventually the peristaltic 

 movements of the muscular lining of the stomach mix the 

 food mass with the acid gastric juice and the food mass passes 

 slowly from the cardiac or front end of the stomach through to 

 the rear or pyloric end. As the food mass becomes acid through 

 the action of the free hydrochloric acid of the gastric juice the 

 action of ptyalin and maltase is completely stopped, the enzymes 

 being killed. The free acid of the gastric juice acts as a germi- 

 cide preventing bacterial putrefaction, and produces an acid 

 medium in which the pepsin of the gastric juice can best act. 

 In the gastric juice gastric rennin coagulates the milk caseinogen 

 preparing it for digestion. Pepsin hydrolyzes protein food 

 to peptones, proteoses, polypeptides and a small amount of 

 amino-acids. Gastric lipase hydrolyzes fats, almost wholly 

 those in the form of emulsions, to glycerol and fatty acids. In 

 the stomach some absorption may take place, though probably 

 small in amount. 



