ANAL CANAL. 1229 



Formerly this terminal portion of the tube was described as the "third part of the rectum," 

 and, like the rest of that bowel, it was supposed to form a reservoir for the retention of the 

 fseces. It is probable that only when the rectum is distended is the superior part of the anal 

 canal occupied by the wedge-shaped lower end of the contained fsecal mass. 



The anal canal begins where the rectum proper terminates, namely, at the 

 level of the levatores ani muscles, opposite the inferior part of the prostate. 

 When the distended rectum is cut across near its inferior end, in a hardened body, 

 and the cavity examined from the interior, a distinct projection, formed by the 

 medial border of the levator ani (O.T. puborectalis, or sphincter recti portion), is seen 

 on each side, indicating the superior limit of the canal. It is said that these ridges 

 can also be felt during life by the finger introduced into the rectum. Below, the 

 anal canal ends at the anus, or anal orifice, by opening on the exterior. 



Its length is usually from 1 to 1J inches (2*5 to 3*7 cm.), being greater when 

 the bowel is empty, and less when it is distended. Its antero-posterior diameter 

 when closed varies between J and J inch (12 to 19 mm.). 



(The direction of the anal canal, as already pointed out, is downwards and 

 posteriorly, often forming an angle of nearly 45 with the horizontal, although 

 it is usually somewhat nearer to the vertical. 

 Relations. It is surrounded by both the external and internal sphincters, 

 and above also by the borders of the levatores ani, these muscles forming a 

 muscular cylinder around it (Fig. 963). On each side is situated the ischio- 

 rectal fossa with its contained fat, which allows of the distension of the canal 

 during the passage of fseces. Posteriorly is placed a mass of mixed connective and 

 muscular tissue, known as the ano-coccygeal body (Symington), which intervenes 

 between it and the coccyx. Finally, anteriorly, it lies close behind the bulb of the 

 urethra and the base of the urogenital diaphragm in the male, and a sound in the 

 urethra can be easily felt by the finger introduced into the anal canal, particularly 

 in thin bodies. In the female it is separated from the vagina by the wedge-shaped 

 mass of fatty and muscular tissue known as the " perineal body." 



Structure of the Rectum and Anal Canal. The wall of the rectum is made up 

 of four coats, viz. : 1. The outer coat, formed in part by peritoneum (already described), 

 and, where the peritoneum is absent, of connective tissue which can be dissected off in 

 several layers. In this connective tissue the hsemorrhoidal vessels run until they pierce 

 the wall of the tube. In it also, at the back and sides of the rectum, are found embedded 

 a number of rectal lymph glands. 



2. The tunica muscularis, which is much thicker than in any other portion of the intes- 

 tine, is composed of two stout layers of unstriped muscle an outer longitudinal and an 

 inner circular like that of the intestine generally. The longitudinal fibres, although 

 present all round, are accumulated chiefly on the front and back of the tube (see p. 1212), 

 where they form two broad bands ; at the sides they are reduced to a thin layer, the 

 deepest fibres of which are folded in and take part in the formation of the rectal valves. 



Where the rectum pierces the floor of the pelvis, the outer layer of longitudinal fibres is united 



the deeper portion of the levator ani, partly by tendinous fibres and partly by an inter- 

 change of muscular fibres, between the levatores and the muscular coat of the rectum. Below, 

 the longitudinal fibres pass between the external and internal sphincter muscles, or through 

 the latter to join the skin around the anus. 



In sagittal sections of the pelvis near the median plane there can generally be seen a distinct 

 band of red, longitudinally arranged, muscular fibres, which descends oil each side from the front 

 of the coccyx to blend with the longitudinal fibres on the back of the rectum. This band is the 

 rectococcygeus muscle. It is composed of striped fibres above, but becomes unstriped below. 



Some unstriped muscular fibres which are found descending in the subcutaneous tissue of the 

 lower part of the anal canal, to join the skin around the anus, have been described by Ellis as the 

 corrugator cutis ani. According to Roux, they are some of the longitudinal fibres of the rectum 

 which have passed through the internal sphincter to the submucous tissue, and then descended 

 to the skin. The front of the rectum at the perineal flexure is, in the male, connected to the 

 of the membranous urethra by a band of muscle, termed the recto-urethralis. 



Jh 



back 



The circular fibres form, along the whole length of the tube, a continuous layer, which 

 doubled inwards to assist in the formation of each rectal valve, and is thickened 

 below to form the internal sphincter of the anus. The sphincter ani internus, as just 

 pointed out, is formed by a great, and rather sudden, increase of the circular muscular 

 fibres, which begins at the superior end of the anal canal. It surrounds the canal for 

 about an inch (2-5 to 3*0 cm.), and terminates at its junction with the skin. 



