23 

 The Vertebrate Body (Cont.) 



Nourishment, Respiration, and Distribution 

 in the Frog and Man 



The Digestive System. One of the primary concerns of the frog, as 

 of all animals, is the obtaining of food. This is not too much of a prob- 

 lem in this instance, for the frog is very fond of insects, such as nice, fat. 

 juicy flies, and there are certainly plenty of these available. The tongue 

 forms an excellent fly trap; it is hinged onto the mouth at the front 

 rather than at the back, as most tongues are, and it is covered with a 

 sticky mucus. When a fly ventures too close the frog flicks its tongue 

 out with amazing rapidity, the insect sticks to it, the tongue flicks back 

 into the mouth, the frog swallows and calmly resumes his attitude of 

 watchful waiting. 



As the struggling fly reaches the throat the muscles carry it down to 

 the esophagus, which carries it on to the stomach by means of peristaltic 

 contractions. The stomach is lined with gastric glands that secrete hy- 

 drochloric acid and pepsin; the pepsin is an enzyme which begins the 

 digestion of proteins, but it cannot function without the hydrochloric 

 acid. The fly breaks up into smaller pieces due to the action of these 

 secretions combined with the muscular action of the stomach walls which 

 churn the food around vigorously. We are sometimes embarrassed by 

 the gurgling sounds resulting from similar churning actions of our own 

 stomachs when we are in polite company. When this phase of digestion 

 has progressed sufficiently, the pyloric sphincter, which guards the exit 

 from the stomach, opens and some of the food is squeezed into the upper 

 end of the small intestine, the duodenum. Here the pancreatic juice 

 from the pancreas, the bile from the liver, and the intestinal juices from 

 the glands in the intestinal wall mix with the food. There are three 

 enzymes in the pancreatic juice — a fat-splitting one, a starch-splitting 

 one, and a protein-splitting one. The intestinal juices contain enzymes 

 which split complex sugars into simple sugars. The bile is partly a 

 waste product from the liver, but it also assists in digestion by break- 

 ing the fats into tiny droplets. It is an emulsifier. 



Meanwhile the food has been churned up by segmentation move- 

 ments of the intestine and has been gradually moved along by the peri- 



330 



