THE ABDOMINAL CAVITY AND CONTENTS 109 



of connective tissue. The muscular layer is actually a 

 double layer, the inner fibers arranged in a circular man- 

 ner, while the outer ones are extended lengthwise of the 

 tube. By alternate contractions and expansions of these 

 muscles the food is forced along the tube to the stomach. 

 In this action the effect produced is similar to what hap- 

 pens when a ring slightly smaller than the tube is forced 

 along a rubber tube; it forces any bodies inclosed in the 

 tube in the same direction that the ring is moving. The 

 contractions of the circular and longitudinal bands of 

 muscles necessary to this action in the guUet have 

 received the name peristalsis. Through peristalsis of the 

 esophagus walls the food is always forced toward the 

 stomach, regardless of the position of the body. The 

 food undergoes no chemical change in either the pharynx 

 cavity or esophagus. 



The Abdominal Cavity and Contents. 



This large body cavity (see p. 29, and plate I), incloses 

 the greater part of the alimentary canal. (See "Ex. 

 XXXIII.) It is separated from the chest at the top by a 

 muscular partition called the diaphragm, while its base is 

 formed of the basketlike bones of the pelvis. At the back 

 are the spinal column and the lower ribs, while the sides 

 and front are formed of thick sheets of muscle without 

 bone support. This cavity is lined throughout with a 

 very smooth membrane called the peritoneum. This 

 peritoneum is so arranged that it not only lines the cavity 

 but also, by extensions, forms folds which support the 

 various parts of the alimentary canal. These folds are so 

 arranged that the parts of the canal are hung in it like a 



