Anatomy anus and rectum


Anatomy anus and rectumWe will describe the parts in the order they are met with in the examination.

Anatomy of the anus

The anus is a longitudinal slit-like aperture which becomes oval during defecation. It is situated an equal distance from the tuberosities of the ischii and about one inch anterior to the tip of the coccyx. It is a little more anterior in the female than in the male.

The integument surrounding the anal aperture is darker in color than the surrounding skin, and contains hair-follicles, sweat glands, and sebaceous glands.

The skin surrounding the anal orifice is arrayed in radiating folds, the result of contraction of the corrugator cutis ani muscle.

The anal orifice varies from one to one and three-quarter inches
in length, but may be dilated to a circumference of 4 to 6 inches.

Anal Canal

The anal canal is a narrow slit-like passage which leads from the anal orifice to the lower end of the rectum, and it is tightly closed by the application of its lateral walls to eacho other. Except during defecation, it is about one and one-half inches in length and proceeds upward and forward from the anal margin to join the rectum at the point where the bowl penetrates the pelvic diaphragm.

The anal canal is closely surrounded by muscles which allow its lateral walls to separate from each other only during defecation. The internal sphincter muscle surrounds it ill nearly its whole length. The external sphincter muscle surrounds the anal orifice and the lower part of the anal wall.

The lower part of the anal canal is lined by modified or squamous epithelium, the upper portion being lined by mucous membrane continuous with that of the rectum. The mucocutaneous margin marks a point where, the bowel and the proctodeum invagination in the embryo coalesce.

The line of the junction is marked by crescentic folds about four in number and are known as the anal valves. Above these valves the mucous membrane is thrown into vertical folds, the columns
and sinuses of Morgagni.

Blood Supply

The arteries of the rectum and anal canal are the superior, middle, and inferior hemorrhoidal and occasionally a branch from the middle sacral and vesicle.

The superior hemorrhoidal veins originate in the plexus of the veins
in the submucous tissue of the anal canal and lower part of the rectum. The plexus communicates freely with the circular veins around the anal orifice, and it is through this communication that the portal and systemic venous circulations are connected; a fact which explains the frequency of the combined external and internal hemorrhoids.

This plexus of veins terminates in the superior hemorrhoidal veins which run up under the mucous membrane along with the corresponding arteries and perforate the muscular coat of the rectum a few inches about the anus.

Muscular compression of these veins by prolonged straining during defecation causes congestion of the plexus and may be followed by varicosity. The main trunks of the superior hemorrhoidal veins terminate in the inferior mesenteric which in turn terminte in the portal. Like other veins of the portal system, they have no valves, another fact which tends to cause congestion and may be followed by dilatation. The middle and inferior hemorrhoidal veins return the
blood from the anus and the circumanal region by the way of the internal iliac which goes into the systemic circulation.

Nerve Supply

The anus and the lower part of the anal canal are liberally supplied with sensory nerves which are derived from the sacral plexus. The rectum has a very poor sensory nerve supply. The external sphincter muscle, which is the most important from a surgical standpoint, is composed of circular and longitudinal fibers.
The circular fibers are more superficial, and they entirely surrond the lower end of the anal canal. Its nerve supply is derived from the third and fourth sacral and the superficial branch of the internal pudic and a filament of the fourth and fifth sacral known as the lesser sphincterian nerve. This nerve is very important in the administration of local anesthesia for relaxation of the external sphincter muscle.

The anal valves are richly supplied with sensory nerves and therefore very sensitive to pain. The mucous membrane above these valves is poorly supplied with nerves, and for this reason the in
jection into internal piles, which are above the anal valves, gives practically no discomfort; a very important factor to remember in the injection treatment of internal piles.

Lymph Vessels

The lymph vessels from the anal margin and the lower part of the anal canal below the pectinate line pass into the subinguinal lymph glands in two ways. Some run anterolaterally across the urogenital triangle; others pass posterolaterally around the lateral aspect of the thigh about the level of the great trochanter.

From the inguinal group they pass through the crural ring to join the external iliac glands. The lymph vessels from the anal canal above the pectinate line and lower part of the rectum drain into a small group of glands on the the posterior surface of the rectum and from there ascend along with the superior hemorrhoidal artery and terminate in the sacral and lumbar lymph glands.