THE MECHANICS OF DIGESTION 999 



the crop and the double stomach. The former appears as an enlarge- 

 ment of the proximal segment of the esophagus, and serves as a 

 reservoir for the food, performing a function similar to that of the oral 

 pockets of the squirrels and" allied animals. Besides, this pro-stomach 

 furnishes a secretion which institutes a swelling of the kernels and a 

 destruction of their cellulose investments. Of special interest is the 

 fact that this organ also secretes a milk-like fluid which serves as food 

 for the young during the first two or three weeks of their life. It 

 contains a considerable amount of fat which is derived from the des- 

 quamated and degenerated epithelial lining. The stomach of these 

 animals consists of two segments, namely, a glandular pro-ventriculus, 

 and a muscular ventriculus. The former furnishes an acid secretion 

 rich in pepsin, whereas the latter reduces the food into smaller frag- 

 ments. In this function it is aided very materially by the solid sub- 

 stances, such as granules of sand, which these animals are in the 

 habit of ingesting with their food. 



The alimentary canal of the mammals presents as its two principal 

 characteristics the division of the stomach into two or four cavities, 

 and the varying length and caliber of the small and large intestines. 

 The carnivora are characterized by a preponderance of the small 

 intestine, and the herbivora by a preponderance of the large intestine. 

 Some of the mammals, such as the rodents and cetacese, are in possession 

 of a stomach consisting of two pouches, while that of the ruminating 

 animals consists of four compartments. In the latter, the esophagus 

 terminates in a vestibular enlargement which communicates with the 

 first and second gastric cavities. The food enters chiefly the first 

 cavity, where it is iAtei mingled with older material and is in part 

 forced into the second compartment. After 30 to 70 minutes (cow), 

 small amounts of the now somewhat softened material are projected 

 into the mouth to be remasticated. Most of this material is finally 

 converted into a liquid mass which upon being reswallowed is directed 

 into a muscular furrow through which it attains the third and fourth 

 cavities. Its still unreduced portion is retained in the first compart- 

 ment to be remasticated if necessary at the rate of 6 to 8 times in the 

 course of 24 hours, each act of mastication lasting from 45 to 60 min- 

 utes. Liquids, on the other hand, may enter all four compartments 

 simultaneously. The capacity of the cow's stomach varies between 

 160 and 230 liters, four-fifths of which are apportioned to the first 

 two chambers. 



The alimentary canal contains secretory as well as muscular ele- 

 ments which are held together by varying amounts of connective tissue. 

 Its length varies considerably in different animals, being shortest in 

 the carnivora and longest in the herbivora. In general, the ratio 

 between its length and that of the entire body is, in man, as 1 : 5 or 1 : 6; 

 in the dog, as 1:6; in the cat, as 1 : 4; in the cow, as 1 :20, and in the 

 sheep, as 1 :27. The mucous membrane lining th^ digestive tract 

 presents a surprisingly large surface to the simplified foodstuffs. 



