440 



APPLIED ANATOMY. 



or, as it has been called, the "white line of Hilton." The external sphincter sur- 

 rounds the lower part of the canal and stretches in a spindle shape from the tip of the 

 coccyx to the central point or tendon of the perineum. Anteriorly it blends with the 

 fibres of the levator ani and the other muscles of the perineum. It is a thick, power- 

 ful, voluntary muscle and extends outward from the white line of Hilton or muco- 

 cutaneous junction. 



Mucous Membrane. The upper half of the mucous membrane of the anal canal 

 has six or eight longitudinal ridges or folds called the columns of Morgagni or Glis- 

 son. Between the lower ends of these columns are small hollows called the crypts of 

 Morgagni, and the free' edges of the mucous membrane guarding the crypts are the 

 anal valves. 



BLOOD-VESSELS. 



Arteries. The rectum and anal canal are supplied by the superior, middle, and 

 inferior hemorrhoidal, and middle sacral arteries (Fig. 446) . 



The supei tor hemorrhoidal artery is the terminal branch of the inferior mesen- 



Inferior mesenteric artery 



Superior hemorrhoidal 



Middle sacral artery 



Middle hemorrhoidal 



from anterior branch 



of the internal iliac 



Inferior hemorrhoidal 

 from the internal pudic 



Rectum 



Uterus 



Bladder, upper surface 



Symphysis 



FIG. 446. The blood supply of the rectum. 



teric. It descends in the pelvic mesocolon until it reaches the rectum, when it divides 

 into two lateral branches. These descend on its surface to about its middle, when 

 they subdivide into six or eight branches which pierce the muscular coat and descend 

 in the submucosa, one beneath each column of Morgagni. At the lower end of the 

 rectum and anal canal they anastomose with the terminal branches of the middle 

 and inferior hemorrhoidal arteries. 



The middle hemorrhoidal arteries , one on each side, come from the anterior branch 

 of .the internal iliac. They descend on the lower part of the rectum and supply the 

 posterior portion of the bladder and vagina, or prostate and seminal vesicles, the 

 lower anterior half of the rectum and upper part of the anal canal, and anastomose 

 with the superior hemorrhoidal branches above and the inferior hemorrhoidal below. 



The inferior hemorrhoidal arteries, two or three on each side, are given off from 

 the internal pudic while in Alcock's canal, at the outer posterior portion of the ischio- 

 rectal fossa ; they pass inward and downward to supply the outer surface of the 

 levator ani and internal and external sphincters and lower portion of the rectum and 



