876 A MANUAL OF ANATOMY 



vessels, it descends within the root of the pelvic meso-colon as low as the 

 third sacral vertebra, where the rectum commences. Here it divides into 

 two branches, right and left, which descend upon the sides of the rectum. 

 From each of these divisions six or more branches are given off, which 

 enter the rectal wall near its centre. Having pierced the muscular coat, 

 they enter the submucous coat, and there subdivide into small terminal 

 branches, which descend in the submucous coat as low as the anal valves, 

 some of them lying within the columns of Morgagni. As these terminal 

 branches descend they give off twigs, which, by anastomosing with branches 

 of the middle and inferior hemorrhoidals, give rise to an arterial anastomotic 

 network in the submucous coat. The disposition of the two primary divisions 

 of the superioi* hemorrhoidal artery on the sides of the rectum is to be borne 

 in mind in the performance of Whitehead's operation for the removal of 

 hemorrhoids. 



The middle hemorrhoidal arteries are two in number, right and left, and 

 are branches of the anterior divisions of the internal iliacs, often arising 

 in common with the inferior vesicals. Having reached the sides of the 

 rectum about its centre, they divide into branches, some of which supply the 

 muscular wall, whilst others enter the submucous coat to take part, along 

 with the superior and inferior hemorrhoidals, in the anastomotic network. 



The inferior hemorrhoidal arteries are two in number, right and left, and 

 each may be a single artery, or there may be two or three on each side. In 

 any case, they are branches of the internal pudic, immediately after that 

 vessel has taken up its position in Alcock's canal on the outer wall of the 

 ischio-rectal fossa. Having pierced the wall of Alcock's canal, and traversed 

 the fat of the ischio-rectal fossa, they approach the wall of the anal canal, 

 where they break up into branches, some of which supply the muscular wall, 

 including the external sphincter and adjacent fibres of the levator ani, whilst 

 others pass into the submucous coat, where they take part, along with the 

 superior and middle hemorrhoidals, in the anastomotic network already 

 referred to. The middle and inferior hemorrhoidal arteries of one side 

 anastomose with their fellows of the opposite side. 



The rectum also receives twigs from the middle sacral and sciatic arteries. 



Veins. — The veins, all of which are destitute of valves, form two rich 

 plexuses, internal hemorrhoidal, situated in the submucous coat, and external 

 hemorrhoidal, lying along the exterior of the rectum, both being confined to 

 its lower third. The internal hemorrhoidal plexus, situated in the submucous 

 coat, receives its radicles from a set of vessels, termed anal veins, which com- 

 mence beneath the anal integument. These anal veins ascend in the wall 

 of the anal canal, some of them lying within the columns of Morgagni, and 

 they are reinforced by other veins from the anal wall. Having given rise by 

 their communications to the internal hemorrhoidal plexus over the lower 

 third of the rectum, the blood is conveyed away from the plexus in two 

 channels as follows : (i) the veins from the upper part of the plexus pierce the 

 wall of the rectum, and open into the external hemorrhoidal plexus ; and (2) the 

 veins from the lower part of the plexus pass through the external sphincter 

 to end in a plexus on the outer surface of that muscle, in which the inferior 

 hemorrhoidal veins originate. The external hemorrhoidal plexus is situated 

 on the outer wall of the rectum over its lower third, and is continuous below 

 with the plexus on the outer surface of the external sphincter. It is from 

 this external hemorrJ: '^idal plexus that the hemorrhoidal veins, which corre- 

 spond with the hemorrhoidal arteries, pass off. The superior hemorrhoidal 

 vein leaves the plexus in two divisions right and left, which ultimately join to 

 form one vessel, the destination of which is the inferior mesenteric vein. It 

 is, therefore, an indirect tributary of the vena portae. The middle hemor- 

 rhoidal veins, right and left, terminate in the internal iliac veins. The inferior 

 hemorrhoidal veins, right and left, pass to the internal pudic veins. Through 

 means of the external hemorrhoidal plexus a free communication is estab- 

 lished between the systemic and portal venous channels, and, there being no 

 valves, when the portal circulation is obstructed the condition known as 

 hemorrhoids frequently results. 



