22 ESSAY ON THE DESCRIPTIVE ANATOMY OF THE 
more or less capacious, exist in the membrane lining the large 
intestine. The difference in degree of vascularity gives rise to 
difference in the colour of the mucous coat in various portions 
of the gut: thus, that lining the caecum is generally more 
deeply coloured than that of the colon, whilst the rectal mucous 
membrane is more vascular, and hence redder than the colic or 
csecal one. 
At the termination of the ileum is the ileo-colic or ileo-csecal 
valve, which is constituted of two folds of mucous membrane, 
almost parallel to each other, and horizontal, leaving between 
them an eliptical orifice when partially drawn asunder. The 
folds consist of the circular fibres of the intestine, lined on the 
inner or ileac side by the villous membrane of the small, whilst 
on the csecal and colic side they are covered by the mucous 
membrane proper to the large intestine. It is worthy of notice, 
that though muscular fibres partly enter into the construction of 
the valve, its efficiency is explicable on purely mechanical 
grounds, as proved by the fact, that it is competent in the dead 
body. 
The anus is the outlet of the intestine, which is perfectly 
closed, except during the evacuation of feculent matters, and is 
made perceptible externally by the elevation of the tail, being 
- situated in a space bounded superiorly by the sacrum and 
coccyx, laterally by the ischial tuberosities, and inferiorly by 
the urethra in the male and vulva in the female. 
It is lined within by the mucous membrane of the rectum, 
which is loose and of a marked red colour. Its external cover¬ 
ing is of common integument, destitute of hairs. Lying be¬ 
tween the skin and mucous membrane are two circular muscles, 
whose office it is to keep the anus closed and prevent con¬ 
stant evacuation of faeces, whilst there are other muscular 
appendages situated externally to these, destined either to 
elevate or retract the anus, being evidently antagonistic to the 
sphincters. 
The internal sphincter ani is in contact with the attached 
surface of the intestinal mucous membrane, and separated from 
the integument by the external one. It is constituted of the 
pale circular fibres of the gut, but towards its free edge certain 
coloured fibres are apparent on it. 
The external sphincter is situated outside the internal one, 
and within the anal integument: it is circular, and composed of 
red fibres, attached superiorly under the first coccygeal bone, 
and inferiorly its fibres blend in the male subject in the accele¬ 
rator urinse and triangularis penis, and in the female with the 
constrictor vaginae. 
The levatores ani are two pale muscles, attached on each side 
