THE DIGESTION OF FEEDS 29 



the milk passes directly into the fourth stomach, where it is 

 curdled by the rennin and subjected to the action of pepsin. The 

 latter ferment acts only in an acid medium and on protein sub- 

 stances, which it causes to break up into soluble compounds, known 

 as proteoses and peptones. Since the ptyalin of the saliva acts 

 only in an alkaline solution, its action on the starch of the feed 

 is stopped when the feed reaches the fourth stomach and is mixed 

 with the gastric juice. 



From the fourth stomach the feed passes through a valve, 

 called the pylorus, into the small intestine. This is a long, tortuous 

 tract, about 120 feet long in cattle, in which three different diges- 

 tive fluids are secreted or emptied : The pancreatic juice, the bile, 

 and the intestinal juice. 



The pancreatic juice is secreted by the large gland called pan- 

 creas (or sweetbread) and is emptied into the small intestine near 

 its upper end, through a duct leading from the pancreas. ThiM 

 digestive fluid contains three specific ferments: Trypsin, amylop-* 

 sin, and lipase. Trypsin converts protein into soluble compounds, 

 mainly peptones, but also compounds of simpler molecular struc- ' 

 ture than those resulting from pepsin digestion, viz., amino acids. 

 Amylopsin changes starch into sugar, and lipase (formerly called 

 steapsin) acts upon the fats, splitting these up into their com- 

 ponent parts, free fatty acids and glycerin (see p. 23). 



The bile plays an important part in the digestion of fats. It is 

 a strongly alkaline, yellowish-green digestive fluid secreted by the 

 liver and stored in the gall-bladder attached to the same. The bile 

 acts upon the fats of the feed that are still unchanged, emulsifying 

 these; i.e., separating them into very minute drops or globules 

 that may be either absorbed through the intestinal wall or readily 

 acted upon by the. fat-splitting ferment lipase of the pancreatic 

 juice. It also aids in the absorption of the fatty acids. The bile 

 contains a number of characteristic components whose importance in 

 the digestion of feeds is not clearly understood, but it has several 

 regulative and digestive functions besides those mentioned; it acts 

 as a natural laxative and prevents an accumulation of waste ma- 

 terials in the intestines, changing poisonous decomposition products 

 of protein into harmless compounds that are excreted through the 

 kidneys. 



The intestinal juice is secreted by numerous small glands in 

 the mucous lining of the intestines, especially in the lower part of 

 the tract. 



