258 DIGESTIVE SYSTEM. 



coli — that voluminous dilatation of gut between the termination 

 of the ileum and mouth of the coecum — and soon expands into a 

 cavity of greater dimensions than even that of the stomach it- 

 self; having attained which prodigious bulk, it begins to con- 

 tract, and continues to do so gradually during its course around 

 the ccecum, until it has completed its second flexure, where it 

 grows so small, that it scarcely exceeds in caliber one of the small 

 intestines ; and though from about the middle of this turn it 

 again swells out by degrees, it never afterwards acquires its for- 

 mer capaciousness : indeed, previously to its junction with the 

 rectum, it once more diminishes, and finally assumes the caliber 

 and general appearance of that gut. Its first flexure has three 

 longitudinal bands, which give it a plicated appearance exter- 

 nally, like the coecum, and form it into very many deep and ca- 

 pacious cells within : its last turn, however, has but two, and the 

 cells in it are not only less numerous, but are much shallower as 

 we approach the rectum. This fact tends much to strengthen 

 our opinion of the uses of these cells : for in this part of the ali- 

 mentary canal the matters being feculent, no farther absorption is 

 required to be made from them ; therefore, of course, they need 

 not be longer detained. Not only, however, are the cells fewer and 

 less distinct at this part, but their supply of blood is diminished ; 

 so that the intestinal secretion (which it is believed contributes to 

 the completion of the digestive process) is here probably wanting 

 altogether, or but very sparingly produced. 



RECTUM. — At the upper part of the circumferent margin of 

 the pelvis, the colon terminates in the rectum. In the iiorse, this 

 is a comparatively short gut, being continued in nearly a straight 

 line to the anus. 



Capacity. — It will hold about three gallons of fluid. 



Peculiarities. — The rectum, independently of its general figure 

 and dimensions, differs from the coecum and colon in possessing 

 but a partial peritoneal covering, and in having no muscular bands, 

 nor cells. Its posterior extremity, more capacious than the ante- 

 rior part of it, is furnished with a circular muscle — the sphincter 

 ani : which, with the adipose matter in which it is cushioned, 

 forms and gives that prominence to the anus so remarkable in the 

 living animal. The use of the sphincter is, by keeping the anus 

 closed, to retain the feculent matter until so much of it be accu- 

 mulated in the rectum as to excite a desire to discharge it. So 

 that the sphincter is a muscle that is in constant action, other- 

 wise the faeces would be continually escaping, and so far it acts in- 

 voluntarily ; but in order to expel them, the animal has recourse 

 to a voluntary power — to abdominal compression, exerted prin- 

 cipally by the internal oblique and transverse muscles ; whose 



