THE ALIMENTARY CANAL 



175 



lumen. Larj>:er folds extend out from the submucosa carrying along 

 these smaller folds. The submucosa does not form the wide ruga- 

 like extensions, as in the urodeles, l)ut it is present between the 

 adjacent epithelial surfaces of the \'illus-like folds. The circular 

 coat of the muscularis is not \'ery thick, and the longitudinal coat 

 is still thinner. 



The Reptilian Small Intestine. — In reptiles there are numerous 

 longitudinal folds of the mucosa and submucosa. The muscularis 

 mucosa may be present as a scattered region of circular cells, or the 

 submucosa and tunica ])ro])ria may form a single narrow connective- 

 tissue region. The circular layer of the muscularis is the thickest 



Fig. 112. — Photograph of a cross-section of the small intestine of the frog, showing 

 villus-like longitudinal folds. Bloodvessels are showni in the supporting mesentery. 



coat; the longitudinal coat is \'ery thin. Mesothelium covers the 

 thin adventitia. 



In the lower vertel)rates the intestine shows little or no marked 

 histological differentiation at the \arious levels, but in the anterior 

 portion the submucosal folds are more numerous, the lumen smaller, 

 and the wall pr()])orti()nally thicker. 



The Small Intestine of Mammals.— The same four coats noted in 

 the stomach form the wall of the small intestine, but here the 

 muscularis is quite regular, with an outer longitudinal and an inner 

 circular layer. (Fig. 113.) The mucosa possesses characteristic 

 features that deserve closer attention. The inner surface of the 

 canal generally shows circular and oblique folds involving the 



