This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The superior hemorrhoidal artery is the terminal branch of the inferior mesenteric. 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.
Fig. 446. - The blood supply of the rectum.
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 ischiorectal 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 anal canal. They anastomose with the middle and superior hemorrhoidals. They are distributed more to the posterior portion of the lower part of the rectum and anal canal while the middle is distributed more to its anterior portion.
The middle sacral artery passes down in the median line from the bifurcation of the aorta to the tip of the coccyx, where it ends in Luschka's gland. It gives a few branches to the rectum at its upper part but they are supposed not to go deeper than the muscular coat. It anastomoses with the superior hemorrhoidal.
The veins of the rectum and anal canal accompany the corresponding superior, middle, and inferior hemorrhoidal arteries. They form two plexuses, an internal submucous plexus and an external plexus on the surface of the rectum. The internal plexus in the submucous coat begins at the anus in fine venous capillaries which pass upward, mainly in the columns of Morgagni, where they form small dilatations or pools and unite into larger branches which pierce the muscular walls about the middle of the rectum to empty into the main superior hemorrhoidal veins and thence into the inferior mesenteric.
The inferior hemorrhoidal veins receive branches from the anus and outer surface of the sphincters and levator am muscles and pass thence to the internal pudic veins.
Fig. 447. - Lymphatics of rectum. (Gerota).
The middle hemorrhoidal vein drains the blood from the external hemorrhoidal plexus on the outer surface of the lower half of the rectum and empties into the internal iliac. It anastomoses with the superior hemorrhoidal above, at about the middle of the rectum, and the inferior hemorrhoidal below, at the upper portion of the anal canal. It is thus seen that the interior of the lower half of the rectum is drained by the superior hemorrhoidal and its exterior by the middle hemorrhoidal. The blood from the upper part of the anal canal drains into the superior hemorrhoidal, that from its lower part into the inferior hemorrhoidal. The blood from the superior hemorrhoidal veins empties into the portal system through the inferior mesenteric, and the blood from the middle and inferior into the general venous system through the internal pudic, internal iliac, and inferior cava. These veins are usually regarded as being without valves, though the opposite view is held by some.
According to Poirier and Cuneo there is a superior group accompanying the superior hemorrhoidal vessels and draining the mucous membrane of the anal canal and rectum and terminating in the nodes of the pelvic mesocolon after traversing the pararectal lymph-nodes; also a middle group partly communicating with the above through the pararectal lymph-nodes while the remainder accompany the middle and inferior hemorrhoidal vessels and drain the lower part of the anal canal above the white line. A third group comes from the skin of the margin of the anus and drains into the inguinal nodes. The pararectal (anorectal of Gerota) nodes may become enlarged in cases of nonmalignant ulcer and can be felt in the region of the ampulla by the finger introduced through the anus, thus leading to a mistaken diagnosis of carcinoma (Fig. 447).
The anus is supplied by the inferior hemorrhoidal branch of the internal pudic nerve, which, as shown by Hilton, crosses the ischiorectal space on the outer surface of the levator ani muscle and passes between the internal and external sphincters to emerge between them at the white line, from whence it sends filaments up on the mucous membrane and down on the skin. This explains the great sensitiveness of the region. It also supplies the external sphincter, hence the association of spasm with pain.