370 THE ABDOMEN AND PELVIS 



to the internal sphincter and some of their fibres are prolonged 

 downwards in the wall of the anal canal. The external sphincter 

 lies very superficially, and is attached to the tip of the coccyx 

 behind and to the central point of the perineum in front. 



The submucous coat of the anal canal is very lax and 

 contains tributaries of the haemorrhoidal veins. 



The mucous coat of the anal canal is loosely attached to 

 the muscular wall and, in its upper part, is thrown into a number 

 of vertical folds, termed the rectal columns (of Morgagni). These 

 are united at their lower ends by small semilunar folds, which 

 are termed the anal valves. Ball suggests that anal fissure is 

 produced by the tearing downwards of one of the anal valves 

 by a scybalous mass. 



The line formed by the anal valves, sometimes termed 

 the pectinate line, marks the junction between the columnar 

 epithelium of the intestinal canal and the scaly epithelium of 

 the skin, and therefore indicates the site of the embryonic anal 

 membrane (p. 380). 



Blood-supply of the Rectum and Anal Canal. (i) The superior 

 hfzmorrhoidal artery (p. 341) descends in the root of the pelvic meso-colon 

 and divides into right and left branches, which run downwards on the muscular 

 coat of the rectum. They give off numerous branches, which pierce the 

 muscular coat and supply the mucous membrane of the rectum and the upper 

 part of the anal canal, anastomosing freely with one another and with the 

 inferior and middle hsemorrhoidal arteries. 



(2) The middle h&morrhoidal arteries arise from the hypogastric arteries 

 and are mainly distributed to the muscular coat of the 'rectum. As they 

 approach the gut they are embedded in cellular tissue, which offers some 

 resistance when the surgeon endeavours to drag down the rectum in the 

 operation of excision (p. 382). This cellular tissue is termed by Ball the 

 lateral ligament of the rectum. 



(3) The inferior hcemorrhoidal arteries arise, on each side, from the internal 

 pudendal in Alcock's canal (p. 373) and run downwards and medially across 

 the ischio-rectal fossae to reach the sides of the anal canal. They are dis- 

 tributed to the muscular wall of the anal canal and to the lower part of its 

 mucous coat. 



(4) The middle sacral artery arises from the abdominal aorta near its 

 termination and runs downwards in the median plane on the anterior surface 

 of the sacrum. It supplies a few twigs to the muscular coat of the rectum. 



Veins of the Rectum and Anal Canal. The internal hcemorrkoidal 

 plexus lies in the submucous tissue of the anal canal, and by its means the 

 superior, middle, and inferior hsemorrhoidal veins communicate with one 

 another. 



The superior hezmorrhoidal veins arise from the upper end of the plexus 

 and pass upwards in the submucous coat of the rectum. About the middle 

 of the rectum they pierce the muscular coat and unite to form a common 

 trunk, which ascends in the pelvic mesocolon and ultimately becomes the 

 inferior mesenteric vein (p. 321). 



The middle hcemorrhoidal veins also arise in the internal haemorrhoidal 



