1232 THE DIGESTIVE SYSTEM. 



only this lower portion of the anal passage is formed from the proctodseum in the embryo. The 

 junction of the skin with the mucous membrane is indicated by a fine wavy line (" white line " 

 of Hilton ano-cutaneous line of Hermann) which runs around the bowel at the level in- 

 dicated. The mucous membrane of the region immediately above the anal valves is of a more 

 or less transitional nature ; glands are absent from it, and over the columnse rectales it is 

 covered with stratified epithelium, the superficial cells of which are flattened, whilst in the 

 grooves between the columns the epithelium is columnar. In the upper zone of the anal canal 

 the mucous membrane gradually approaches to the rectal type, but the intestinal glands and 

 lymph nodules are few and scattered. 



Anus or Anal Orifice. At the inferior aperture of the anal canal, the 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 appear, the last 

 being termed glandulse circumanales. 



Action of the Sphincters. In connexion with the anal canal are found three muscles 

 namely, the paired levatores ani, the external sphincter, and the internal sphincter the action 

 of which may be briefly referred to here. 



Levator Ani. The fibres of the levator which arise from the pubis (pubo-coccygeus 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 slightly towards the pubes. There is little 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 narrowed or " pinched 

 in " to form the 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 faeces when 

 the rectum is distended. It is probably relaxed during defaecation, except perhaps at the com- 

 pletion of the act. The muscle is under the control of the will. 



The m. sphincter ani externus forms a muscular cylinder around the inferior two-thirds of the 

 anal canal, with (except in the case of some of its inner fibres) an anterior and a posterior attach- 

 ment. 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 inferior and greater part. 

 It is under the control of the will, but under ordinary circumstances it is in a state of tonic 

 contraction. 



The m. sphincter ani interims is merely a thickening of the circular muscular coat at the inferior 

 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 faecal 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 hsemorrhoidal arteries ; to these another less important, though constant, 

 source may be added the middle sacral artery. 



1. The superior haemorrhoidal artery, the principal artery of the rectum, is the prolongation 

 of the inferior mesenteric. At first it descends in the root of the pelvic mesocolon 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 posteriorly, 

 the left more anteriorly, and the two, as it were, embracing the bowel between them. 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 of the rectal columns. These terminal branches give 

 off numerous twigs in their course, which form a haemorrhoidal plexus in the submucosa by 

 anastomosing with one another, and also with branches of the middle, and, in the inferior part of 

 the bowel, of the inferior haemorrhoidal artery. 



2. The middle haemorrhoidal arteries, two in number one on each side are usually branches 

 of the hypogastric or of the internal pudendal ; they run on the wall of the inferior part of the 

 rectum, and each breaks up into four or five small branches, some of which supply the muscular 

 wall of the inferior part of the rectum, whilst the others pierce the muscular coat near the 

 superior end of the anal canal, and join in the submucosa with the plexus formed by the 

 superior haemorrhoidal artery already described. 



3. The inferior hsemorrhoidal arteries, generally two or three in number on each side, arise 

 at variable levels from the internal pudendal. They are distributed to the levatores ani 

 and the sphincters. Other branches pierce the sphincters and break up in the submucosa into a 

 close network which supplies the inferior part of the anal canal, and communicates above with 

 the plexus formed by the superior and middle haemorrhoidal arteries. The inferior haemor- 



