334 THE VERTEBRATE BODY 



chewing performs several important functions besides making the food 

 particles small enough to swallow. It mixes the food with the saliva, 

 secreted by the salivary glands, which contains a starch-splitting enzyme, 

 ptyalin. This makes a starchy food such as a cracker taste sweet if it is 

 chewed long enough. The saliva also dissolves sugar and salt, a neces- 

 sary preliminary to taste, since the taste buds on the tongue are stimu- 

 lated only by substances in solution. The saliva also contains a sticky 

 substance called mucin, which helps in swallowing by holding the par- 

 ticles of food together and by "greasing" them so that they will slide 

 down the esophagus easily. 



The human small intestine is not only much longer in proportion to 

 body size than that of the frog, but it is also lined by millions of finger- 

 like villi, each of which contains a double series of vessels, the lymph 

 vessels or lacteals, which pick up and transport most of the digested 

 fats, and the blood capillaries which do the same for the absorbed sugars 

 and amino acids. 



The human ileum enters the large intestine from the side leaving a 

 blind sac, the caecum, extending beyond this connection. On the tip of 

 the caecum there is a slender, worm-like process, the well-known vermi- 

 form appendix, a vestigial organ which frequently becomes inflamed 

 and abscessed. There is no cloaca in man after birth — -the digestive 

 system has a posterior opening, the anus, which is separate from the 

 openings of the other organs. There is a cloaca in the human embryo, 

 however, but during embryonic development a partition separates the 

 urogenital ducts from the termination of the large intestine and separate 

 openings are formed before the infant is born. 



Glottis 

 ■ L arynx 



Alveoli • 



Fig. 23.4. The respiratory system of the frog. 



The Respiratory System. The frogs are land animals so they have 

 lung respiration, but it is not as efficient as that of more advanced animals 



