DISSECTION OF THE ABDOMEN. 319 



from the rest of the intestinal mass. After the serous and muscular 

 coats have been observed on the inflated csecum, the bowel should be 

 slit open on the convex side of its crook, the incision being extended to 

 its point. The mucous surface is to be gently washed ; and, in con- 

 nection with its study, the student is to examine the two orifices found 

 on the concave side of the crook. 



Structure of the Large Intestine. Throughout nearly the whole of 

 its length, the wall of the large bowel is made up of four coats, similar 

 to those of the small intestine. 



1. The Serous Coat is derived from the peritoneum, but it forms here 

 a less complete investment than in the case of the small intestine, con- 



Fig. 42. 



Diagrammatic View (magnified) of a small Portion of the Mucous Membrane of the 

 Colon (Allen Thomson). 



A small portion of the mucous membrane cut perpendicularly at the edges is shown in perspective ; 

 on the surface are seen the orifices of the crypts of Lieberkuhn or tubular glands, the most of them 

 lined by their columnar epithelium, a few divested of it and thus appearing larger ; along the sides 

 the tubular glands are seen more or less equally divided by the section ; these are resting on a wider 

 portion of the submucous tissue, from which the blood-vessels are represented as passing into the 

 spaces between the glands. 



siderable areas of the wall being without this covering. Thus, it is 

 absent where the caecum and double colon adhere to the pancreas and 

 abdominal parietes in the sublumbar region ; it is also absent where 

 these two intestines adhere to each other, and where the parallel 

 portions of the double colon come into contact ; and lastly, as will be 

 seen in the dissection of the pelvis, the terminal part of the rectum is 

 without a peritoneal covering. 



2. The Muscular Coat consists of two distinct layers — an external longi- 

 tudinal and an internal circular. Throughout nearly the whole extent 

 of the large intestine the longitudinal fibres are not uniformly distri- 

 buted over the wall, but are collected into distinct bands, the areas 

 between the bands being provided only with circular fibres. When 

 these bands contract they shorten the intestine, and throw the wall of 



