

CHAP. XXV.] THE TUNICS OF THE INTESTINAL CANAL. 223- 



fat, and presents the same characters throughout the whole intes- 

 tinal canal. Placed immediately underneath the mucous mem- 

 brane, it constitutes the medium, through which the various san- 

 guiferous and other vessels and the nerves pass to that membrane. 



The muscular coat of the small intestine consists of two layers 

 or planes which differ from each other as regards the direction of 

 their fibres. The external plane is composed of longitudinal fibres, 

 continuous superiorly with the longitudinal fibres of the stomach ; 

 they form a continuous tunic surrounding the intestine, and extend- 

 ing from the pylorus to the coecum. C. B. Albums'* states that 

 they exist only as a band a finger-breadth broad, and corresponding 

 to the concave border of the intestine along which the mesentery is 

 attached; and to this he attributes the concavity which the inflated 

 intestine presents towards the mesentery. We have no doubt, 

 however, that the longitudinal fibres form a continuous tunic 

 around the intestine, though they are strongest along the line of 

 attachment of the mesentery, and are very apparent in that situa- 

 tion at times when they are indistinct elsewhere. The circular 

 fibres are much more distinct than the longitudinal ; the direction 

 of which they cut at right angles. They surround the intes- 

 tine in a circular manner, not spirally, as some anatomists have 

 asserted. 



The muscular tunic of the large intestine is likewise disposed in 

 two layers of fibres. The external, however, does not, as in the 

 small intestine, form a uniform layer around the intestine, but is 

 developed chiefly in three bands, about half an inch wide, with a 

 few intervening longitudinal fibres. These bands commence at the 

 root of the vermiform appendix of the coecum, and extend in this 

 form to the rectum where they become expanded and form a con- 

 tinous tunic over the whole intestine. The longitudinal bands are 

 shorter than the intestine; the effect of this is to produce a pucker- 

 ing of all its coats at certain intervals throughout the whole length 

 of the colon. At these points the colon appears to be constricted, 

 as by a shorter bundle of circular fibres, and its mucous membrane 

 projects into the interior, forming large folds. These folds separate 

 sacculated portions of the intestine, which are the cells of the colon, 

 and the convex bulgings seen on the exterior of the inflated large 

 intestine are the walls of these cells. The circular fibres are ar- 

 ranged in the same manner as in the small intestine, being spread 

 uniformly over the surface of the intestine. 



* Specimen Anatomicum exhibens novam tenuium Hominis Intestinorum 

 Descriptionem. Ludg. Bat. 1724. 



