370 



HOW FOOD IS PREPARED FOR BODY USE 



What conditions are necessary for the digestion of protein ? What 

 is the effect of an extreme heat and cold on the action of hydrochloric 

 acid and pepsin with a protein? Make a table giving all results of 

 the above tests of conditions necessary for the digestion of protein. 



Chewing and swallowing. After food has been chewed and 

 mixed with saliva, it is rolled into little balls and pushed by the 

 tongue into such position that the muscles of the throat cavity 

 may seize it and force it downward. Food, in order to reach the 

 gullet from the mouth cavity, must pass over the glottis which is 

 the opening into the trachea. When food is in the process of 

 being swallowed, the upper part of the gullet forms a trapdoor 

 over this opening. When this trapdoor, called the epiglottis, is not 

 closed, and food " goes down the wrong way," we choke, and the 

 food is expelled by coughing. 



The esophagus. After food leaves the mouth cavity, it gets 

 beyond our direct control, and the muscles of the gullet, stimulated 

 to activity by the presence of food in the tube, push the food down 



by a series of slow-moving 

 muscular contractions until it 

 reaches the stomach. These 

 wavelike movements, peri- 

 stalsis, occur also in the stomach 

 and the small intestine. Peri- 

 staltic movement is caused by 

 2) muscles which are not under 

 voluntary nervous control, al- 

 though anger, fear, disgust, 

 or other unpleasant emotions 

 may slow them or even stop 

 them entirely. 



Stomach of man. The 





E 



_^5""^^™__ 



££ 



-g — J > Js*i*&£-2-2-g- 



Food passes through the digestive tract by stomach is a pear-shaped organ 



means of a series of successive wavelike move- , , » , , ,. , , ., 



ments, which are under the control of the Capable 01 holding aDOUt three 



nervous system. The constricted portion is _:_i Q Onnrnsitp in tViP oiillpt 



always preceded by an area of relaxation. P lntS ' opposite t O tne gUliet, 



the end which empties into 

 the small intestine is provided with a ring of muscle called the 

 pylorus (pi-lo'r#s). When this muscle relaxes, it permits the 



