THE ANAL CANAL. 1045 
modified skin of its lower zone passes into the ordinary skin. <A little way 
outside the orifice, hairs, sebaceous glands, and large modified sweat-glands 
(glandule circumanales) appear. 
Action of the Sphincters.—In connection with the anal canal are found three muscles— 
namely, the paired levatores ani, the external sphincter, and the internal sphineter—the action 
of which should be briefly referred to here. 
Levator Ani.—The fibres of the levator which arise from the pubis (pubo-coceygeus or 
sphincter recti portion) pass backwards on each side of the beginning of the anal canal, and, in 
great part, meet behind the passage. These two muscular bands—which are but a little distance 
apart at their origin, and are actually united behind the bowel—are closely approximated 
during the contraction of the muscles, like the limbs of a clamp, and, pressing on the sides of the 
anal canal, they assist’ in closing the upper part of that passage, whilst at the same time drawing 
it sughtly towards the pubes. There is ttle doubt that the levator ani in this way acts as one 
of the chief sphincters of the bowel; and it should be noticed that it is placed where its action 
would be most effective, namely, opposite the point at which the rectum is uarrowed or “ pinched 
in” to formthe anal canal. In addition to its sphincter action the muscle supports the expanded 
bowel immediately above the anal canal, and in this way sustains the weight of the faces when 
the rectum is distended. It is probably relaxed during defecation, except perhaps at the com- 
pletion of the act. The muscle is under the control of the will. 
The external sphincter forms a muscular cylinder around the lower two-thirds of the anal 
canal, with (except in the case of some of its inner fibres) an anterior and a posterior attachment. 
When the muscle contracts, its fibres are tightly stretched between its two attachments, and the 
space between them is reduced to a narrow antero-posterior slit. By this action the anal canal is 
flattened from side to side and closed, so that, whilst the levator ani is the sphincter of the upper 
aperture of the anal canal, the external sphincter closes its lower and greater part. It is under 
the control of the will, but under ordinary circumstances it is in a state of tonic contraction. 
The internal sphincter is merely a thickening of the circular muscular coat at the lower end 
of the bowel. It is continuous with the circular fibres of the gut, not only in structure, but 
probably also in action, its chief use being to empty the anal canal completely, after the passage 
of each feecal mass. Owing to the fact that the canal is an antero-posterior slit, not a circular 
orifice, and that the internal sphincter forms a muscular ring around it, acting alone, it is scarcely 
competent to keep the sides of the canal in apposition, and probably it acts rather as a detrusor 
than a true sphincter of the anal passage. 
Vessels.—The rectum and anal canal receive their blood supply from three chief sources, 
namely, the three hemorrhoidal arteries; to these another less important, though constant, 
source may be added—the middle sacral artery. 
1. The superior hemorrhoidal, the principal artery of the rectum, is the prolongation of the 
inferior mesenteric trunk (Fig. 566, p. 795). At first it descends in the root of the pelvic meso- 
colon until the rectum is reached. Here it divides into two chief branches which run downwards 
and forwards around the sides of the rectum—the right, usually the larger, lying more behind, 
the left more to the front, and the two, as it were, embracing the bowel between them (Fig. 701). 
From these two arteries come off secondary branches (about five to eight in all), which pierce the 
muscular coat about the middle of the rectum, and then descend in the submucosa as a series of 
longitudinally-running “ terminal branches” as far as the anal valves, above the level of which 
one is usually found beneath each column of Morgagni. These terminal branches give off 
numerous twigs in their course, which form a hemorrhoidal plexus in the submucosa by 
anastomosing with one another, and also with branches of the middle, and, in the lower part of 
the bowel, of the inferior hemorrhoidal artery. 
2. The middle hemorrhoidal arteries, two in number—one on each side—are usually 
branches of the anterior division of the internal iliacs; they run down on the side wall of 
the lower part of the rectum, and after giving off branches to the bladder, seminal vesicles, 
prostate (or vagina, according to the sex), each breaks up into four or five small branches, some 
of which supply the muscular wall of the lower part of the rectum, whilst the others pierce the 
muscular coat near the upper end of the anal canal, and join in the submucosa with the plexus 
formed by the superior hemorrhoidal artery already described. 
3. The inferior hemorrhoidal arteries, generally two or three in number on each side, arise 
at variable levels from the internal pudic arteries, whilst these latter are situated on the outer 
side of the ischio-rectal fosse. They pass inwards and downwards through the fat in the 
fossee, and, near the wall of the anal canal, break up into branches, some of which are distri- 
buted to the levatores ani and the sphincters, whilst others pierce the sphincters and break up in 
the submucosa into a close network which supplies the lower part of the anal canal, and com- 
municates above with the plexus formed by the superior and middle hemorrhoidal arteries. 
The inferior hemorrhoidal artery is distributed chiefly on the back, and the middle hemorrhoidal 
chiefly on the front of the lower part of the bowel. 
4. One or more small branches of the middle sacral artery reach the back of the rectum, 
where they are distributed chiefly, if not solely, to the muscular coat. 
Anastomosis of the Hemorrhoidal Arteries.—The superior and middle hemorrhoidal arteries 
anastomose freely, not only in the hemorrhoidal plexus of the submucosa, but also, as a rule, by 
a few large branches on the exterior of the bowel: some perforating branches of the middle 
sacral also join the plexus in the submucous layer at the lower part of the rectum, as do numerous 
small branches from the inferior heemorrhoidal arteries which pierce the sphincters. In addition, 
