The pelvic diaphragm, pelvic cavity, and ischioanal fossae are
complex structures beyond the scope of this tutorial. In practice, the
general surgeon (in the ambulatory setting), will deal with much more
superficial anatomy. Therefore, a more superficial approach to the anatomy
of this area will be presented.
The anal canal (approximately 2.3-3.5 cm long) is the terminal
part of the large intestine, extending from the upper aspect of the pelvic
diaphragm to the anus. Surrounded by internal and external sphincters, it
descends posteroinferiorly between the anococcygeal ligament and the perineal
The internal anal sphincter is an involuntary sphincter supplied
by parasympathetic fibers which pass through the pelvic splanchnic nerves.
This sphincter relaxes in response to pressure (gas or feces) distending the
The external anal sphincter is a voluntary sphincter innervated
primarily by S4 through the inferior rectal nerve.
Within the anal canal, the uppermost portion of the canal is
dominated by a series of mucosal ridges called anal columns. These columns
contain the terminal branches of the superior rectal artery and vein, and it is
here (at their superior ends) that the division of the rectum from the anus is
The Pectinate Line
The Pectinate Line (also known as the Dentate or Mucocutaneous
Line) is a clinically important landmark due to the fact that it is visible and
approximates the level of certain anatomic changes that will be discussed below.
It lies at the inferiormost level of the anal columns and indicates the junction
of the superior part of the anal canal (derived from the embryonic hindgut) and
the inferior part (derived from the embryonic proctodeum). It is
this difference in embryonic origin which gives rise to differing arterial,
venous, and nervous connections (above and below this line)
Blood Supply and Nerves
The superior portion of the anal canal (that is, superior to the
pectinate line) is supplied by the superior rectal artery. Below the
pectinate line, the inferior rectal arteries supply the inferiormost part of the
anal canal. Between the two, the middle rectal arteries form anastomoses
with each to assist in the blood supply.
A plexus of veins branch around the anal canal. Above the
pectinate line, this internal rectal plexus drains into the superior rectal
vein. Below, the plexus drains into the inferior rectal vein.
Similar to the arterial system, a middle rectal vein exists, which mainly drains
the muscularis externa, and anastomoses with the superior and inferior veins.
Although not pictured, the lymphatic drainage differs above and
below the pectinate line. Above it, the lymphatics drain into the internal
iliac lymph nodes. Below, the vessels drain into the superficial inguinal
Lastly, the anal canal also has differing nervous innervations
above and below the line. Above the pectinate line, the nerve supply is
visceral, coming from the inferior hypogastric plexus. As is it visceral,
this part of the anal canal is only sensitive to stretch. Below the
pectinate line, the nerve supply is somatic, receiving its supply from the
inferior rectal nerves (branches of the pudendal). As it is somatically
innervated, it is sensitive to pain, temperature, and touch.
||Superior, middle, inferior rectal arteries
||Superior, middle, inferior rectal veins
||Inferior hypogastric plexus, rectal nerve plexus, inferior