THE ABDOMEN 787 



Structure of the Intestinal Canal. 



Small Intestine. — The wall of the small intestine, which is 

 cylindrical, is composed of four coats — serous, muscular, sub- 

 mucous, and mucous. 



The serous coat is formed by peritoneum derived from the mesen- 

 tery proper. In the case of the duodenum it is incomplete, but it 

 forms a complete investment to the jejunum and ileum, except 

 along a narrow interval corresponding to the mesenteric border 

 of the bowel, where the peritoneal investment becomes continuous 

 with the two layers of the mesentery proper. 



The muscular coat {mitscidaris externa) is composed of plain 

 muscular tissue, disposed in two layers, external or longitudinal, 

 and internal or circular. The external or longitudinal fibres are 

 continuous with the corresponding fibres of the stomach, and they 

 are best marked along the an ti- mesenteric border. The internal or 

 circular fibres are continuous with the outermost fibres of the 

 sphincter pylori, and form a much thicker layer than the longitu- 

 dinal. The muscular coat .attains its greatest thickness in the 

 duodenum, whence it gradually diminishes. Between the two 

 muscular layers there is a gangliated plexus of non-medullated 

 ner\'e-fibres, called the plexus myentericus of Auerbach, and also a 

 plexus of Hinphatic vessels. 



The submucous coat is situated between the muscular and 

 mucous coats. It is composed of loose areolar tissue, and serves 

 partly as a connecting medium and partly as a bed in which the 

 arteries subdivide. It contains a gangliated plexus of non-medul- 

 lated nerve-fibres, called the plexus of Meissner, and a plexus of 

 l5nnphatic vessels. In the duodenum this coat lodges Brunner's 

 glands, and the deep ends of the solitary glands project into it 

 throughout. 



The mucous coat is red and thick in the upper part of the small 

 intestine, but pale and thin in the lower part. It is covered by a 

 single layer of columnar epithelium, many of the cells being mucus- 

 secreting goblet cells. The protoplasm of the cells is longitudinally 

 fibrillated, and their free ends are covered by a delicate striated 

 basilar border, the striations being vertically disposed. Underneath 

 the epithelium there is a basement membrane, known as the stib- 

 epithelial endothelium, and underneath this is the main part of 

 the mucous coat, which is essentially composed of adenoid tissue, 

 that is to say, retiform tissue containing in its meshes lymph 

 corpuscles. At the deepest part of the mucous coat, and forming a 

 part of it, there is a stratum of plain muscular tissue, called the 

 muscularis mucosce {muscidaris interna), which in many situations 

 is disposed in two layers — outer longitudinal, and inner circular. 



