298 



DIGESTION AND ABSORPTION 



What a Gland Does. Enzymes. — In man there are the saliva 

 gland of the mouth, the gastric glands of the stomach, the pancreas 

 and liver, the two latter connected with the small intestine, and the 

 intestinal glands in the walls of the intestine. Besides glands which 

 aid in digestion there are several others of which we will speak 

 later. As we have already learned, a gland is a collection of cells 



which takes up material 

 /ood tube from within the body and 



manufactures from it some- 

 thing which is later poured 

 out as a secretion. An 

 example of a gland in plants 

 is found in the nectar- 

 secreting cells of a flower. 



Certain substances, 

 called enzymes, formed by 

 glands cause the digestion 

 of food. The enzymes 

 secreted by the cells of the 

 glands and poured out into 

 the food tube act upon 

 insoluble foods so as to 

 change them to a soluble 

 form. The}^ are the prod- 

 uct of the activity of the 

 cell, although they are not 

 themselves alive. We do 

 not know much about 

 enzymes themselves, but 

 we can observe what they 

 do. Some enzymes render soluble different foods, others work 

 in the blood, still others prol)ably act within any cell of the body 

 as an aid to oxidation, when work is done. Enzymes are very 

 sensitive to changes in temperature and to the degree of acidity or 

 alkalinity ^ of the material in which they act. We will find that 

 the enzymes found in glands in the mouth will not act long in the 

 1 The teacher should explain the meaning of these terms. 



Diagram of a gland. i, the common tube 

 which carries ofif the secretions formed in 

 the cells lining the cavity c ; o, arteries 

 carrying blood to the glands ; v, veins 

 taking blood away from the glands. 



