Chap. XIV] 



THE DIGESTIVE SYSTEM 



255 



(4) The serous coat is derived from the peritoneum. 



The peritoneum. — This is a double membrane, the outer or 

 parietal layer of which lines the inner surface of the abdominal and 

 pelvic cavities, and the inner or visceral layer is reflected back over 

 the contained organs. The arrangement of the peritoneum is 

 very complex, for several elongated sacs and double folds extend 

 from it, to pass in between and either wholly or partially surround 

 the viscera of the abdomen and pelvis. One important fold is 

 the omentum, which hangs like a curtain in front of the stomach 

 and the intestines ; another is the mesentery, which surrounds the 

 greater part of the small intestine. The posterior portion of the 

 mesentery is gathered into folds which are attached to the spine 

 and serve to hold the intestines in place. 



Functions of the peritoneum. — Like all serous membranes 

 the peritoneum serves to prevent friction between contiguous 

 organs by secreting serum which acts as a lubricant. It also serves 

 to hold the abdominal and pelvic organs in position, to unite and 

 separate these organs, and supports numerous nerves and blood- 

 vessels. The omentum usually contains fat, and, in addition to 

 the usual functions, serves to keep the organs it covers warm. 



Divisions of the alimentary canal. — For convenience of descrip- 

 tion, the alimentary canal may be divided into : — 



Mouth, containing tonsils, tongue, salivary glands, and teeth. 



Pharynx. 



(Esophagus. 

 Stomach. 



r Duodenum. 

 Small or thin intestine < Jejunum. 



[ Ileum. 



f Caecum. 



Large or thick intestine • 



f Ascending. 

 Colon \ Transverse. 



i Descending. 

 Rectum. 



MOUTH, OR BUCCAL CAVITY 



The mouth is a nearly oval-shaped cavity with a fixed roof 

 anteriorly, a flexible roof posteriorly, and a movable floor. It 



