510 



VESSELS OP INTESTINES. 



eluded by the serous membrane with the exception of the points 



of attachment of the omenta. 

 Fig. 152.* The descending portion has mere- 



ly a partial covering on its an- 

 terior surface. The transverse 

 portion is also behind the perito- 

 neum, being situated between the 

 two layers of the transverse me- 

 so-colon, and has but a partial 

 covering. The rest of the small 

 intestine is completely invested 

 by it, excepting along the con- 

 cave border to which the mesen- 

 tery is attached. The ccecum is 

 more or less invested by the peri- 

 toneum, the more frequent dispo- 

 sition being that in which the in- 

 testine is surrounded for three- 

 fourths only of its circumference. 

 The ascending arid the descend- 

 ing colon 'are covered by the se- 

 rous membrane only in front. 

 The transverse colon is invested 

 completely, with the exception of the lines of attachment of the 

 greater omentum and transverse meso-colon. And the sigmoid 

 flexure is entirely surrounded, with the exception of the part cor- 

 responding with the junction of the left meso-colon. The upper 

 third of the rectum is completely enclosed by the peritoneum; the 

 middle third has an anterior covering only, and the inferior third 

 none whatsoever. 



Vessels and Nerves. The Arteries of the alimentary canal, as 

 they supply the tube from above downwards, are the pterygo-pala- 

 tine, ascending pharyngeal, superior thyroid, and inferior thyroid in 

 the neck; ossophageal in the thorax; gastric, hepatic, splenic, superior 

 and inferior mesenteric in the abdomen ; and inferior mesenteric, 

 iliac, and internal pudic in the pelvis. The veins from the abdomi- 

 nal alimentary canal unite to form the vena portse. The lymphatics 

 and lacteals open into the thoracic duct. 



* A vertical section of the parietes of the anus, passing through the middle line of 

 one of the columns of the rectum, and the neighbouring parts, c. The internal 

 sphincter, with its arched fibres transversely divided. D, D. The plane of arched fibres 

 of the muscular coat, similarly divided. E. The point of greatest contraction of the in- 

 ternal sphincter. F. The external sphincter. G. The point of greatest contraction of 

 the same muscle. H. The plane of longitudinal fibres of the muscular coat, longitudi- 

 nally divided, i. Some of these fibres terminating in the internal sphincter, k. Others, 

 terminating in the external sphincter. 1. The remaining longitudinal fibres, collected 

 into a semitendinous fasciculus, passing over the lower margin of the internal sphincter, 

 to be reverted upward within the duplicature of the column, m. These reverted fibres 

 again becoming muscular, and terminating in the mucous coat. L. The mucous coat. 

 n. A bristle in one of the sacs. G. 



