COATS OF SMALL INTESTINE. 457 



be seen shining through the fibrous layer, when the muscular coat has 

 been taken away. 



The pieces of intestine may be opened and washed to show the mucous 

 coat, but the gut should be cut along the line of attachment of the mesen- 

 tery, so as to avoid Peyer's glands on the opposite side. 



Mucous coat. The lining membrane is thicker and more vascular at 

 the beginning than at the ending of the small intestine. It is marked by 

 numerous prominent folds (valvulae conniventes) ; and the surface of the 

 membrane is covered with small processes (villi) like the pile of velvet. 

 Occupying the substance of the mucous coat are numerous glands ; and 

 covering the whole is a columnar epithelium. 



A thin layer of non-striated muscular fibres (muscularis mucosce) covers 

 the outer surface of this coat (fig. 154, c?), and sends inwards prolonga- 

 tions between the tubules into the villi. 



The valvulce conniventes (fig. 150) (valves of Kerkring) are permanent 

 ridges of the mucous membrane, which are arranged circularly in the 

 intestine, and project into the alimentary mass. Crescentic in form, 

 they extend round the intestine for half or two-thirds of its circle, and 



Fig. 150. 



THE DUODENUM OPENED SHOWING THE VALVUL^: CONNIVENTES, AND THE OPKNI.NG OF THE BILE 

 DUCT. The duct of the paucreas is also represented in greater part of its course. 



a. Duodenum. d. Pancreatic duct. 



b. Pancreas. e. Opening of the common duct in the intestine 



c. Common bile duct. (Henle). 



some end in bifurcated extremities. Larger and smaller folds are met 

 with, sometimes alternating ; and the larger are about two inches long, 

 with one-third of an inch in depth towards the centre. Each is formed 

 of a doubling of the mucous membrane, which incloses vessels between 

 the layers. 



They begin in the duodenum, about one or two inches beyond the py- 

 lorus, and are continued in regular succession to the middle of the jeju- 

 num ; but beyond that point they become smaller and more distant from 

 one another, and finally disappear about the middle of the ileum, having 

 previously become irregular and rudimentary. The folds are largest and 

 most uniform beyond, and not far from the opening of the bile duct. 



The aperture of the common bile and pancreatic ducts (fig. 150, e) is a 



