178 THE DIGESTIVE SYSTEM 



a diffuse lymphoid tissue, and solitary lymph nodules are relatively 

 common. In the ileum, near the jejunum or lower, the lymph 

 nodules form groups, called Peyer's patches, which may extend 

 into the submucosa. Xo villi co\er some nodules which project 

 into the lumen. The muscularis mucosa may be of varying dis- 

 tinctness with an outer longitudinal and inner circular layer of 

 smooth muscle. 



The submucosa of loose fibroelastic tissue carries the larger 

 blood and lymph vessels, Meissner's nerve plexus and, except 

 in the region of the duodenum, has no glands. Branched tubular 

 glands, Brunner's glands, appear in the submucosa of the upper 

 region of the duodenum and often extend o\'er into the adjoining 

 region of the pyloric stomach. The secretory portion of these glands 

 is composed of pyramidal or columnar epithelium. The secretion 

 contains a proteolytic enzyme similar to pepsin in its action. Ducts 

 from these glands lead up through the mucosa and open either 

 between the villi or into the crypts of Lieberkiihn glands. The cells 

 lining the excretory ducts are similar to those of the epitheliiun 

 lining the duodenum. 



Outside the submucosa is the muscularis coat consisting of an 

 inner sheath of circularly disposed smooth muscle and an outer 

 longitudinal coat. Connective tissue containing Auerbach's plexus 

 lies between them. 



The adventitia is composed of a thin layer of fibroelastic connec- 

 tive tissue covered with mesothelium. 



The Large Intestine.— The Large Intestine uf Elasmubranchs. ^The 

 large intestine of Selachians consists of the so-called spiral valve, 

 in which the inner surface of the tube is a spiral meml)ranous shelf 

 formed by an extension of submucosa covered with a mucosa. An 

 adventitia with superficial cuboidal epithelium forms the external 

 coat. Internal to this are scattered strands of circularly arranged 

 smooth muscle cells, forming a circular coat of a muscularis. The 

 submucosa of fibroelastic connective tissue forms the core of the 

 spiral valve. The mucosa has a mnnber of sim])le tu])ular iuAolu- 

 tions resembling glands but they are lined with a stratified or pseudo- 

 stratified columnar epithelium. The epithelium covering the mucosa 

 not so invaginated is of the same type as that found between the 

 folds. The folds help to increase the absorption of material passing 

 over the valve but have no secretory activity. Lymphoid and 

 myeloid tissue may be consjucnous in the connective tissue below 

 the epithelium. 



