522 



VERTEBRATE LIFE AND ORGANIZATION 



sceral peritoneum 

 Lon^itu-dinaL muscle 



Circular muscle 

 Submucosa. 



Mucos 



Villus 



Villus 



Mucous 



secreting 

 ola.nd. 



Capillaries 



Lyrnpha-tic- 

 ve-ssel 



Musculcuns 

 m.ucosa.e 



V&irb 

 Lymphatics'^ ^ 



Arle-rj/ — 



Circular 



muscl e 



LonOitudinal- 

 muscle 



"Intestinal 

 ^lan-d 



•Enzyme 

 seci'etind 

 Cells 



Sabmucosao 



"Mcrve-s 



Visceral pcritoncunx- 



Figure 26.4. A, A cross section of the small intestine of a mammal to show its 

 constituent layers; B, a further enlargement of a block of tissue from the wall of the 

 small intestine. 



The stomach and intestine lie in the peritoneal cavity— the largest 

 division of the coelom— and are covered by the visceral peritoneum. 

 Mesenteries, which support the internal organs and provide a route for 

 blood vessels and nerves, extend from the viscera to the body wall. The 

 outer fibers of the muscular coat are usually described as longitudinal; 

 the inner as circular. Actually both layers are spiral; the outer is an 

 open spiral and the inner a tight spiral. The relaxations and contrac- 

 tions of these layers are responsible for the peristaltic and churning 

 movements. The mucosa consists of a layer of smooth muscle, con- 

 nective tissue, and finally the simple columnar epithelium next to the 

 lumen. In the small intestine of mammals and birds, the mucosa bears 



