82 SPECIAL HISTOLOGY. 



the exception of a small portion of the rectum, the walls of the 

 alimentary canal consist everywhere of three tunics : a serous — 

 the peritonaeum ; a muscular — consisting of two or even three 

 layers; and a mucous membrane, the latter containing a great 

 number of glandular structures, which may be divided into 

 three groups, racemose mucous glands, tubular glands, and 

 closed follicles. 



§ 147. 



The peritoneum is much thicker in its external or parietal, 

 than in its internal or visceral layer (in the former case O04 — 

 0'06'", in the latter 002 — 0-03'") though its structure is 

 essentially the same in each locality. It consists principally of 

 connective tissue with distinct, variously interwoven bundles, and 

 abundant reticulated elastic fibres, which are coarser in the 

 parietal lamina. A loose subserous connective tissue, contain- 

 ing more or less fat, unites the peritonaeum with the other organs, 

 or, as in the mesenteric folds, connects its layers together ; under 

 the visceral lamina, however, it is very little developed except 

 in certain localities [colon, appendices epiploicae), and in cer- 

 tain folds of the peritonaeum, it does not exist at all. The 

 free surface of both lamellae of the peritonaeum is lined by a 

 simple tesselated epithelium, whose slightly flattened, polygonal, 

 nucleated cells measure, on the average, 0*01 "'; they are so 

 closely united and so constantly kept moist, that the free 

 serous surface appears perfectly smooth and shining. 



The peritonaeum is, in general, but scantily supplied with 

 vessels; they are most abundant in the omenta, in the visceral 

 layer and in the subserous tissue, in which last alone, lym- 

 phatics have as yet been found. The nerves are also but 

 few, and are especially to be met with in the omentum, the 

 mesenteria, and hepatic ligaments, where they accompany the 

 arteries, 



§ 148. 



Muscular tunic of the alimentary canal. — The whole alimen- 

 tary tract, from the stomach to the rectum, possesses a special 

 muscular coat, which, however, does not everywhere present 

 the same conditions. 



In the stomach the muscular tunic varies in thickness ; at 



