38 



A PRIMER OF BIOLOGY 



the animal and the enzyme-forming glands which 

 open into it (Fig. 14). Into the mouth-cavity three 

 chief kinds of organic food materials enter and are 

 there masticated and mixed to- 

 gether, viz., proteids, carbohydrates, 

 and fats (p. 28). There, also, they 

 are mixed with saliva, and the 

 starchy constituents are, to a cer- ' 

 tain extent, acted on by the 

 ptyalin present in that secretion. 

 The mixed food is then swallowed 

 and transferred to the stomach, by 

 whose rhythmic contractions it is 

 again mixed with secretions derived 

 from glands in its walls. The chief 

 constituent of the secretion of the 

 gastric glands is pepsin, and by it 

 the proteids are attacked. After a 

 period of gastric digestion, the food 

 passes on to the intestine in whose 

 FIG. 14. Diagram walls another series of glands, intes- 

 of the alimentary tinal glands, occur, which also add 

 canal and chief digestive secretions to the mixture, 

 Mf^mouth^Sa' one ' es P eciall y changing cane sugar 

 salivary gland- into glucose and fructose. Further, 

 G, gullet; St, certain special glands, connected 

 Stomach; L, liver; w jth the intestine by means of 



smar^Sine 1 ; 8 P ecia J ****> * their secretions. 



Li, large intestine. One or these glands is the pancreas 

 which has in its secretion a fer- 

 ment which attacks any starch left unacted upon 

 by the ptyalin, another which changes cane sugar 

 into grape sugar and fruit sugar, another which 

 attacks proteids and yet another which acts on fats. 

 Another important gland is the liver, which con- 



