23 
ABDOMINAL VISCERA OF THE HORSE. 
of the first coccygeal bones, and spreading downwards and for¬ 
wards on to the reclum, form an attachment for the internal 
sphincter, and blending with the longitudinal fibres, so as to 
increase the thickness of the muscular coat of the rectum. The 
action of these muscles must be that of elevating the anus and 
shortening the rectum from before backwards. 
The retractors proper to the anus are one on each side 
attached to the inner surface of the articular extremity of the 
ischium. Extending from before backwards, and rather upwards, 
they blend with the external sphincter. Their action is obviously 
that of retracting the anal opening. 
Vessels , Nerves , and Lymphatics of the Intestine. 
The intestinal canal, as a whole, receives arterial blood from 
the anterior and posterior mesenteric arteries, hepatic branch of 
the coeliac axis, with branches from the internal pudic. The 
arteries of the small intestine are derived from the anterior 
mesenteric, whose divisions, varying from twenty-four to 
twenty-eight, proceed to the small intestine, with the exception 
of four, which minister to the nutrition and functions of the 
large intestine. The brandies extending from the main trunk, 
at acute angles, proceed between the layers of the mesentery, to 
within one and a half or two inches from the gut, where they 
anastomose, forming vascular arches, from which the secondary 
branches arise, and, proceeding on to the intestine, ramify on the 
several coats, especially the mucous one. The anterior division 
of the anterior mesenteric artery, proceeding to the duodenum, 
anastomoses with the duodenal branch of the hepatic artery. 
The last iliac division inosculates with the caecal and colic 
branches of the same trunk. 
The caecum and colon receive arterial blood solely from the 
branches derived from the anterior mesenteric, with a slight 
contribution from the posterior mesenteric arteries. The 
branches of the former originate opposite the flexure made by 
the caecum and colon. The caecal divisions, two in number, pro¬ 
ceed downwards and forwards till they reach the gut. The 
posterior one passes round the posterior part of the border of the 
caecum, to get on the under surface of the latter, extending to the 
apex in somewhat a straight course, and ramifying collaterally; 
at its termination it forms a vascular network, by anastomosis 
with the superior caecal artery. The latter one, reaching the 
gut, extends directly forwards towards the apex, and comports 
itself like the former. Thus we see the flexure formed by the 
caecum and colon is supplied by collateral branches from the 
superior and inferior caecal mesenteric divisions, both these 
v«%. 
