PROBLEM 2. The Digestive System Alakes Food Usable 



Mucous membrane Stomach cavity 



191 



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Sections of capillaries 



Fig. 202 A magnified section through the lining 

 of the stomach. Four tiny glands are shown. 

 What lines each gland? The juice secreted 

 flows into the stomach cavity. 



zyme for the special catalysts that are 

 made in a living organism. 



The enzymes that change food sub- 

 stances into other and simpler substances 

 are called digestive enzymes. Ptyalin is 

 one of these. Many different digestive 

 enzymes are produced in the human 

 body; each digests, that is, changes some 

 particular food substance into simpler 

 compounds. One set of enzymes digests 

 protein, another fat, another starch. 

 Others cause the digestion of the various 

 complex sugars. 



Digestive enzymes are made in other 

 animals and in plants as well as in man. 

 You can discover the effects of a plant 

 enzyme by doing Exercise 3. Man has 

 learned to extract some of the digestive 

 enzymes from living animal and plant 

 cells. In fact, some of them are extracted 

 in such large quantities that they can be 

 bottled and sold. 



Not all enzymes are digestive enzymes. 

 Some make it possible for oxidation to 

 go on and there are manv others that are 

 necessary for the various cell activities. 



Where digestive enzymes are made. 

 When the lining of the stomach or the 

 upper part of the small intestine is ex- 

 amined with a hand lens it is seen to be 

 dotted with pores. Each of these opens 

 into a microscopic bag or pocket sunk 

 into the wall of the digestive tube. In 

 the stomach alone there are approxi- 

 mately 35,000,000 of such pockets. The 

 pockets are lined by cells whose pro- 

 toplasm makes enzymes. We say the 

 protoplasm of these cells secretes (see- 

 creets') enzymes. When they are dis- 

 solved in water this mixture of water and 

 enzymes, together with some other sub- 

 stances secreted by the cells, is spoken 

 of as a digestive juice. As the digestive 

 juice diffuses out of the cells it fills the 

 bag or pocket. The juice then trickles out 

 through the pore into the stomach, or 

 small intestine, as the case may be. Such 

 a group of secreting cells is called a gland. 

 Those in the stomach are called gastric 

 glands. Those in the small intestine are 

 intestinal glands. It is interesting to note 

 that the glands of one part of your body 

 secrete certain products; those of another 

 part secrete different substances. 



Large digestive glands outside the tube. 

 Digestive glands are not always micro- 

 scopic pockets like those just described. 

 Sometimes secreting cells are massed to- 

 gether, forming one large organ com- 

 posed of many microscopic "bags" or 

 "sacs." These tiny bags are clustered to- 

 gether much as the individual grapes 

 might be in a bunch of grapes. As the 

 cells of each bag secrete, the juice flows 



