144 PRINCIPLES OF ANIMAL BIOLOGY 



result of the action of ptyalin is that all of the starch is converted into 

 maltose. Ordinarily, because of the short sojourn of the food in the 

 mouth, little starch digestion actually takes place there, and since 

 ptyahn acts only in an alkahne medium, its action is stopped by the 

 acid of the stomach. The maltose remaining from salivary digestion is 

 acted upon by inverting enzymes in the small intestine and associated 

 organs, to which reference is made below. In the salivary secretion 



. — there is also some albumen and mucin. The latter is changed to mucus 

 which acts as a lubricant for the particles of food. 



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 

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



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



- — ■'chloric acid, two important enzymes, pepsin and rennin, and perhaps a 

 third enzyme of less importance, the gastric lipase. The hydrochloric 

 acid affords a suitable medium for the action of the enzymes (except the 

 lipase), and incidentally stops the action of the ptyalin descending from 

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

 factories 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. 

 Pepsinogen is rendered active (converted into pepsin) by the hydrochloric 

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

 The acidity of the gastric juice is usually estimated at about 0.3 per cent. 

 Pepsin acts only on proteins converting them to peptones and proteoses. 

 The process of absorption of the products of digestion through the gastric 

 mucosa is not very active. Water and some sugar and peptones are 

 absorbed, particularly if the solution be strong. Ordinary fats are not 

 acted upon in the stomach, but emulsified fats (cream) are said by some 

 investigators to be digested by the gastric lipase. A statement of the 

 results of digestion of fats is deferred, however, to a later paragraph. 



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

 pylorus, the acid acts upon a substance in the mucosa of the duodenum 

 and changes this substance to secretin. The secretin is absorbed by the 

 blood and is carried to the pancreas which is thereby stimulated to secrete 

 its fluids. Since secretin is not affected by heat nor by alcohol it is not 

 considered to be an enzyme, but is classified in a group of activators 

 called hormones. That the stimulation of the pancreas is not by means 

 of nerves is proved by the fact that the pancreas functions properly after 

 the branches of the vagus and splanchnic nerves with which it is innervated 

 are cut. 

 . The Pancreatic Juice. — The pancreas produces a thin watery secre- 



/ tion which amounts in quantity to about 500 cc. to 800 cc. per day, or 

 about 1 to 1^ pints. This secretion contains three enzymes, which act 



