THE ALIMENTARY CANAL. 503 



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

 both lamellae of the peritonaeum is lined by a simple tessellated epithe- 

 lium, whose slightly flattened, polygonal, nucleated cells measure, on the 

 average, 0-01 of a line, 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 sub- 

 serous tissue, in which last alone, lymphatics 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 ihefundus 

 it is thin (J-J of a line) ; in the middle, it has a thickness of about } a 

 line ; in the pyloric region, finally, about f or even 1 line. It consists 

 of three incomplete layers : 1, 



most externally, longitudinal Fig. 200. 



fibres, especially at the cardia, 

 where they arise from the ex- 

 pansion of a part of the lon- 

 gitudinal fibres of the oesopha- 

 gus ; and also at the pylorus 

 and in the pars pylorica, 

 whence, tensely stretched, they 

 are continued upon the duo- 

 denum ; 2, circular muscles, 

 in the middle region, from the 

 fundus to the pylorus ; where 

 they are accumulated, consti- 

 tuting the so-called sphincter 

 of the pylorus ; 3, most internally, oblique fibres, which, in connection 

 with the circular fibres, embrace ihefundus as in a sling, and run ob- 

 liquely upon the anterior and posterior walls of the stomach, towards its 

 greater curvature, where they terminate upon the outer surface of the 

 mucous membrane or unite together. 



In the small intestine, the muscular coat is somewhat thicker in the 

 duodenum and the upper portions, than in the lower ; it has, in general, 

 a thickness of J J of a line, and is composed only of longitudinal and 

 transverse fibres. The former are always less developed and do not 



* FIG. 200. Stomach of Man, reduced: a, oesophagus, with the longitudinal fibres; tr, 

 transverse fibres (second layer), for the most part dissected off; tr', transverse fibres of the 

 fundus; o, fibres, obliquce p, pylorus ; d, duodenum. 



