PROBLEM 2. The Digestive System Makes Food Usable 



Fig. 203 How do you think 

 these various kinds of teeth 

 are associated with the dif- 

 ferent kinds of food eaten? 



Toothless (Anfeafer) 



Insect-eating [ArmadiiWo) 



Herbivorous (Horse) 



Meot-eoting (Dog) 



Omnivorous (^tAan) 



vou can swallow when you decide to 

 do so. But, once you have swallowed and 

 the food has entered the lower part of 

 the gullet you cannot control its pas- 

 sage. You can understand why this is 

 true when you learn more about the 

 muscles making up the walls of the di- 

 gestive tract. At the upper end are volun- 

 tary muscles. At the lower part of the 

 gullet and along the rest of the tract the 

 muscles are involuntary. You can control 

 voluntary muscles as you wish. Involun- 

 tary muscles are not under conscious con- 

 trol. They are, however, controlled by 

 other parts of the nervous system; they 

 need messages sent to them before they 

 contract or relax. 



Figures 157 and 159 show that the cells 

 of voluntary and involuntary muscles are 

 quite different in appearance. The volun- 

 tary muscles are often spoken of as 

 striped, or striated (stry'ay-ted), muscles; 

 the involuntary are said to be smooth. 

 What other important difference is there 

 in the appearance of these two kinds of 

 muscles? 



The various muscle fibers which make 

 up the walls of the food pipe lie in rings. 

 One ring contracts after another, thus 

 producing what seems to be a wave run- 

 ning along the tube. The wave is slow 

 but steady. If you have ever watched 

 a worm crawling, you will know how 

 the contracting food pipe looks, for the 

 same thing happens in the worm's whole 

 body. This wave of muscular contraction 

 is called peristalsis (perr-i-stall'sis). The 

 food is caught in this wave of contraction 

 and forced onward toward the stomach. 

 You will hear about peristalsis again in 

 connection with the intestines. 



Digestion in the stomach. If you could 

 look at the inside of your stomach with 

 a magnifying glass at the moment the 

 food arrives there, you would see gastric 

 juice trickling from the microscopic gas- 

 tric glands through the microscopic 

 pores. And if you are enjoying the sight 

 and smell and taste of your food, you 

 would see the juice flowing even more 

 freely. One of our first American sur- 

 geons, William Beaumont, in the early 



