104 PRINCIPLES OF ANIMAL BIOLOGY 



appendix. Highly specialized modifications of the stomach occur in 

 ruminants (animals which chew the cud) and in seed-eating birds. 



Digestion in Man. — Inasmuch as the digestive process as it occurs in 

 man has been much more intensively studied than in any other animal, 

 the discussion of digestion which follows will be based on the human 

 system. In the mouth, food is broken up, during which process the three 

 pairs of salivary glands pour out their secretion (saliva) which is mixed 

 with the food. The saliva contains an enzyme, ptyalin, which is al^le to 

 transform starch, particularly cooked starch, into certain sugars. The 

 breakdown of starch occurs by degrees, the intermediate products being 

 various dextrins, but in no case does the digestion in the mouth go 

 farther than to maltose, which is not one of the simple sugars. It is still 

 a disaccharide (page 40) and not readily diffusible through protoplasm. 

 Ordinarily, because of the short sojourn of the food in the mouth, little 

 starch digestion actually takes place there ; and since ptyalin acts only in 

 an alkaline medium, its action is stopped by the acid of the stomach 

 when the food reaches that organ. 



In the stomach, the food is acted upon by the secretion of the gastric 

 glands which are small branched or simple tubular glands located in the 

 inner layer of the stomach. The movement of the muscles of the 

 stomach mixes the food with the gastric secretion, which contains hydro- 

 chloric acid and two important enzymes, pepsin and rennin. The hydro- 

 chloric acid affords a suitable medium for the action of the enzymes 

 and incidentally stops the action of the ptyalin descending from the 

 mouth. The rennin coagulates milk, a fact made use of in cheese fac- 

 tories where a preparation of rennin made from calves' stomachs is used 

 to separate the curd from the whey. Pepsin as it comes from the gastric 

 glands is in an inactive state in which it is called pepsinogen. Pep- 

 sinogen is activated (converted into pepsin) by the hydrochloric acid, 

 which is secreted in a concentration of about 0.4 to 0.5 per cent. Pepsin 

 acts only on proteins, converting them to peptones and proteoses, 

 which are also proteins but simpler than most proteins taken as food. 

 Ordinary fats are not acted upon in the stomach. 



Absorption of foods in the stomach is negligible. Alcohol is absorbed 

 there, which may account for its quick action on mental and other 

 physiological processes. 



Secretin. — When the acid stomach contents are ejected through the 

 pylorus, the acid acts upon a substance in the lining epithelium of the 

 duodenum and changes this substance to secretin. The secretin is 

 absorbed by the blood and is carried to the pancreas and liver which are 

 thereby stimulated to secrete their fluids. Secretin belongs to a class of 

 activators known as hormones. Normally, the pancreas and liver are 

 also controlled in part by nerve impulses. Nevertheless, these glands dis- 



