THE ABDOMEN 875 



with the plain longitudinal fibres on the posterior wall of the anal 

 canal there are two minute muscles (sometimes united into one), 

 called the redo-coccygei muscles. These spring from the front 

 of the coccyx, and descend upon the posterior wall, where they 

 blend with the longitudinal fibres. 



There is nothing specially noteworthy in the submucous coat. 



The mucous coat presents the columns of Morgagni, and the 

 anal valves of Morgagni. The columns of Morgagni are sLx or 

 more in number, and take the form of permanent vertical folds, 

 which are confined to the upper two-thirds of the anal canal, 

 where they are separated from each other by longitudinal grooves. 

 They are composed of inflections of the mucous membrane, which 

 contain plain muscular tissue belonging to the muscularis mucosae, 

 and two small bloodvessels. They stop short about \ inch above 

 the anus. The anal valves of Morgagni are situated at the lower 

 ends of the colmnns of Morgagni, about \ inch above the anus. 

 They lie in the lower ends of the longitudinal grooves, where they 

 extend between adjacent columns, and they bear a resemblance to 

 the valves which are met with in veins. They are semilunar folds 

 of the mucous membrane, their free margins being directed up- 

 wards, and behind each valve there is a small recess or ' sinus.' 

 The mucous membrane of l^he anal canal is replaced in the lower 

 third (below the anal valves) by modified skin, continuous with 

 that of the exterior. In the upper two-thirds of the canal the 

 mucous membrane is hned by columnar epithelium, and in the 

 upper third it is provided with a few Lieberkiihn's crypts and 

 l5niiphoid nodules. The cutaneous wall of the lower third of the 

 anal canal is covered by modified epidermic epithelium, which is 

 continuous at the anal margin with the epidermis. It is at the 

 margins of the anal valves where this modified epidermic epithelimn 

 gives place to the columnar epithelium of the mucous membrane, 

 and here also the modified skin of the lower third of the canal 

 becomes continuous with the mucous membrane of the upper 

 two-thirds. The point of transition is indicated by a faint hne, 

 which is known as the white line of Hilton. 



Sphincter Ani Internus. — ^This so-called muscle is merely a thicken- 

 ing of the circular plain muscular fibres of the gut. It is confined 

 to the region of the anal canal, and, viewing it as an independent 

 muscle, or rather thickening of muscular tissue, it begins very near 

 the upper end of the anal canal, and terminates about \ inch 

 above the anus. Though auxiliary to the external sphincter, the 

 principal action of the internal sphincter is to expel the contents of 

 the anal canal. 



Blood-snpply of the Rectum and Anal Canal— Arteries.— The rectum and 

 anal canal receive their arterial supply from (i) the superior hemorrhoidal, 

 which is a single vessel, (2) the middle hemorrhoidals, right and left ; (3) the 

 inferior hemorrhoidals, right and left ; and (4) branches from the middle 

 sacral and sciatic arteries. 



The superior hemorrhoidal artery is of large size, and is the direct con- 

 tinuation of the inferior mesenteric. Having crossed the left common iliac 



