SEC. 6. THE STRUCTURE OF THE INTESTINES. 



The Small Intestine. 



257. The intestine, small and large, throughout its length 

 from the pylorus to close upon the rectum, follows in its structure 

 the general plan previously described 208. A thin outer longi- 

 tudinal muscular layer, covered by peritoneum, is succeeded by a 

 thicker inner circular muscular layer, and this double muscular 

 coat is separated by a submucous layer of loose connective-tissue, 

 carrying the larger blood vessels, from the mucous membrane 

 which consists of an epithelium lying upon a connective-tissue 

 basis of peculiar nature, a well-developed muscularis mucosse of 

 longitudinal and circular fibres marking off the mucous membrane 

 proper from the underlying submucous tissue. 



In the small intestine the outer longitudinal muscular layer is 

 evenly distributed over the whole circumference of the tube and 

 is everywhere much thinner than the inner circular layer, which 

 is the more important layer of the two. The individual fibre- 

 cells of these muscular layers of the intestine are large and well 

 developed. In the thin sheet of connective-tissue which separates 

 indistinctly the two layers lies the plexus of Auerbach, a plexus of 

 nerve-fibres, for the most part non-medu Hated, at the nodes of 

 which are gathered groups of very small nerve-cells, the substance 

 of each cell being especially scanty. This plexus supplies the two 

 muscular layers with nerve-fibres. 



The submucous coat contains besides blood vessels and 

 lymphatics, a somewhat similar plexus of nerve-fibres, called the 

 plexus of Meissner; from this plexus fine nerve-fibres proceed to 

 the blood vessels, to the muscularis mucosse, and possibly to other 

 structures. 



258. The Mucous Membrane. This is thrown into folds 

 which are not as in the case of the stomach temporary longi- 



