102 



HUMAN BIOLOGY 



inches long. (See Fig. 89.) Like the rest of the food tube, 

 it is lined with mucous membrane. It has two layers of 

 muscles in its walls, the fibers of one layer running length- 

 wise, and the fibers of the other layer being circular. In 

 swallowing, the food does not fall down the gullet of its 

 own weight, but the circular bands of muscle in front of the 

 food relax, and those behind it contract and pusJi it on into 

 the stomach. This wavelike motion is called peristalsis. 



The stomach, the greatest enlargement of the food tube, 

 is like a large bag lying sideways. It lies to the left 



side of the abdomen. The 

 walls of the stomach con- 

 sist chiefly of muscular 

 fibers which run lengthwise, 

 crosswise, and slantwise, 

 making three coats (Exp. 

 7, also Fig. 95). As soon 

 as the food reaches the 

 stomach, the layers of 

 muscles begin to contract, 

 changing the size of the 

 stomach, first in length, 

 then in breadth, thus 

 churning the food to and 

 fro, and mixing it with the 

 gastric juice, a fluid more 

 active than the saliva. For 

 as the food enters the stom- 

 ach, the mucous membrane lining it turns a bright red, 

 and many little gastric glands in the lining begin to 

 secrete gastric juice. 



Digestion in the Stomach. The stomach churns the 

 food from two to four hours after the meal, according to 



FIG. 95. MUSCULAR AND OTHER 

 LAYERS IN WALL OF STOMACH. 



i, mucous lining ; 2, layer of blood vessels 

 and connective tissue ; 3, muscular 

 layers (involuntary muscles) ; 4, con- 

 nective-tissue fibers. (Peabody.) 



