THE ISTESTINES. 4S5 



destitute of hair, it is rich in sebaceous follicles. "We will only notice the 

 muscles. 



The Sphincter of the Anus {sphincter atii) is formed of circular fibres, 

 some of which are fixed above, under the base of the tail, and are mixed, below, 

 with the muscles of the perineal region, especially the posterior constrictor of the 

 vulva in the female ; in the male, these fibres are lost on the surface of the 

 perineal aponeurosis. Comprised between the skin and the prolongation of 

 the muscular layer of the rectum, this nuiscle is (during life), owing to its 

 tonicity and natural shape, in a state of almost permanent contraction, in order 

 to keep the anal aperture closed ; it is only relaxed during the expulsion of 

 faecal matters or intestinal gases. 



The Retractor of the Anus, or Ischio-anal muscle (retractor ani), is a 

 wide band, attached to the internal surface of the sacro-sciatic ligament, and even 

 to the supra-cotyloid crest, by aponeurotic fibres. The fasciculi composing this 

 band are all parallel to each other, while their posterior extremities are insinuated 

 beneath the sphincter, and are mixed with its fibres. This arrangement of the 

 retractor ani clearly indicates that it pulls the anus forwards, re-establishing it in 

 its normal position after expulsion of the faeces — an act that always results in 

 carrying the posterior extremity of the rectum backwards. 



These two muscles are of a red colour, and belong to those of animal life. 

 Their vessels are derived from the same sources as those of the rectum. The 

 hcemorrhoidal nerve supplies them both with filaments. 



Differential Characters of the Intestines in the other Animals. 



In the domesticated animals, the iutestines vary as much in their dimensions, length, and 

 diameter, as in their general arrangement. 



1. The Intestines of the Rabbit (Fig. 286). 



As for the stomach, so for the intestine we place the Rabbit immediately after Solipeds, 

 because the analogies are more marked in them than in the other animals. 



The mmll intestine does not show any dilatation at its commencement, hut it has an 

 enormous one at its termination in the caecum, and which has in its interior a fine Peyer's patch ; 

 for this reason it is named the glanthdar pouch. The otlu-r agminated glands are equally 

 very developed, and are remarkable for their thickness, though they are not numerous — being 

 only seven or eight. They are easily seen through the thin wall of the intestine, when this 

 has been washed and inflated. This intestine reaches the caecum in passing before the first 

 portion of the colon. 



The caecum is also proportionately more voluminous than in Solipeds. As in them, it has 

 tlie sliape of a cone suddenly attenuated towards its summit, and forming an arch or crook at 

 its upper part; but this erook is not very distinctly separated from the colon, and is continuous 

 with it, without showing any constriction. Its internal surface is very curious, as it is 

 traversed by a spiral ridge of mucous membrane, which describes about twenty turns on the 

 wall of tiie intestine, and ceases about 4 inches from the point. This layer — the piesence of 

 which is marked by a depression externally, that corresponds to its fixed border — may be alx)ut 

 \ of an inch or more in depth. The inferior cul-de-sac of the viscus, into which this fold does 

 not enter, forms, like the termination of the small intestine, a veritable glandular pouch. The 

 orifice of this latter intestine does not project into the interior of the caecum ; Bauhin's valve is 

 a disc, pierced like the iris, and fixed by its larger circumference around tiie margin of that 

 opening. 



The colon also shows the division iuto two portions — the tirst, dilated and sacculated, and 

 even provided with rudimentary longitudinal bauds on its external sur.'ace — is covered inter- 

 nally by round and regularly arranged prominence-! which resemble t!iick villi, or traces of 

 the valvulae conniventes; the .second, narrower and uniformly cylindrical, lies alongside the 

 duodenum, ascends int-i the diaphragmatic rei^'ion in company with that intestine, and termi- 

 nates in the rectum, after describing several floating convolutions. On the sides of the rectum 

 are two elongated glands which open iuto the perineal region. 



