484 TEE DIGESTIVE APPARATUS IN MAMMALIA. 



substances thrown out on the surface of the intestinal tube, it loses its name and 

 receives that of excremejtt or J(eces. This excrement, compressed by the peri- 

 staltic contractions of the muscular tunic, is divided into httle rounded or oval 

 masses, which find their way to the rectum, where they accumulate, and whence, 

 in due course, they are expelled. 



C. Rectum (Fig. 282). 



The rectum extends, in a straight line, from the entrance to the pelvic 

 cavity to the posterior opening of the digestive canal, or amis. It is nothing 

 more, properly speaking, than the extremity of the small colon, the limit which 

 separates them being somewhat arbitrary ; it differs from that viscus, however, 

 in having no bulgings, and in its walls being thicker and more dilatable ; so that 

 it can be distended into an elongated pouch, and form a reservoir or receptacle 

 for the excrementitious matters until they are expelled. 



Relations. — It is related, superiorly, to the roof formed by the os sacrum ; 

 inferiorly, to the bladder, the deferent canals, vesiculse seminales, prostate gland» 

 Cowper's glands, or to the vagina and uterus ; laterally, to the sides of the 

 pelvis. 



Mode of attachment. — There ought to be considered as such : 1. The posterior 

 extremity of the colic mesentery, representing the meso-redum. 2. An orbicular 

 fold, constituted by the peritoneum in its circular reflection around this viscus 

 at the extremity of the pelvic cavity. 3. The suspensory ligaments of the penis, 

 which, joining under the rectum, form a ring encircling the posterior extremity 

 of the intestine (see Fig. 282 and the description of the penis). 4. A thick, 

 triangular band, comprising two lateral parts, and composed of white muscular 

 fibres ; this band, which is really a prolongation of the muscular tunic of the 

 viscus, is detached from the rectum above the anus, and is attached to the 

 inferior aspect of the coccygeal bones, between the inferior sacro-coccygeal 

 muscles, where its outline can be seen beneath the skin when the tail is elevated. 



Stkuctuee. — The serous membrane does not envelop the whole of the 

 rectimi, that portion which traverses the bottom of the pelvic cavity being left 

 uncovered by it. The muscular layer is very thick, and composed of large, 

 longitudinal, and slightly spiral fasciculi, beneath which are annular fibres. The 

 mucous membrane, loosely attached to the muscular tunic, shows longitudinal and 

 transverse rugfe. The siuall mesenteric and the internal pvdic artery {artery of 

 the bidb) supply these membranes with blood. The nerves are derived from the 

 •pelvic or hypoyastric plexus. 



Anus. — The anus, or posterior opening of the digestive tube, is situated 

 at the posterior extremity of the rectum, under the base of the tail, where, in 

 Solipeds, it can be seen forming a rounded prominence, which diminishes with 

 age. It is at the border or margin of this orifice, which is corrugated like the 

 mouth of a draw-purse where the intestinal mucous membrane meets with, and 

 is continued by, the external skin. 



In proceeding from within outwards, there are found the following elements 

 entering into the structure of the anus : 1. The mucous membrane of the 

 rectum. 2. The prolongation of the circular and longitudinal fibres of the 

 muscular layer, forming what is named the internal sphinctei'. 3. A sphincter 

 muscle, composed of red fibres, which receives the insertion of a retractor. 4. The 

 fine, hairless, and closely attached skin, which covers the sphincter ; though 



