Chapter 11


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Chapter 11. ANAL CANAL AND ANUS

11.1 SPHINCTER MUSCLES OF ANAL CANAL

The internal anal sphincter is the caudalmost part of the circular muscle of the rectum. As such, the internal sphincter is composed of smooth muscle fibers which are hypertrophic and form what is commonly called the internal anal sphincter.

The subcutaneous external anal sphincter muscle, composed of skeletal muscle, surrounds the anus and covers the internal sphincter, except caudally where the fibers of the latter muscle are not present. Fibers of the external anal sphincter are attached above by coccygeal fascia to the coccygeal vertebrae, and below to the bulbospongiosus and urethralis muscles in the male, or blend with the constrictor vulvae muscle in the female.

11.2 RECTOCOCCYGEUS MUSCLE

These paired muscles, like the internal anal sphincter, are composed of smooth muscle fibers and are classified as part of the wall of the rectum. In this case the muscle fibers extend out from the longitudinal smooth muscle of the rectum and pass through the fascia attaching external sphincter to the coccygeal vertebrae. The function of the rectococcygeus muscles are to anchor and stabilize the rectum during defecation.

11.3 ZONES OF THE ANAL CANAL

The anal canal is the short, terminal part of the large intestine and is divided into three annular zones: the columnar zone; intermediate zone; and cutaneous zone. As previously mentioned, the external orifice of the anal canal is the anus.

11.4 COLUMNAR ZONE

This, the most cranial zone of the anal canal, is named for the permanent longitudinal folds on the internal surface of the anal canal, called anal columns, which fade out as they are traced cranially but are more pronounced at their caudal ends. The columnar zone is one-half centimeter long in the dog but indistinct in the cat, and is recognized by the darker color of its mucosa.

The epithelium of the zone is stratified squamous. It covers the anal columns and the depressed grooves in between the columns called anal sinuses.

Anal glands are classified as tubulo-alveolar glands. The glandular secretions are stored within their ducts and are deposited on fecal material in the anal canal. They are present in the anal canal of carnivores and swine.

11.5 INTERMEDIATE ZONE

Also called the anocutaneous zone, this narrow area is the demarcation between the columnar zone and the most caudal zone, the cutaneous one one, and is in the form of a scalloped fold.

11.6 CUTANEOUS ZONE

This zone is the most caudal one and joins the skin surrounding the anus. It is approximately 4 cm. wide and covered with cornified stratified squamous epithelium and fine hairs. The circumanal (perianal) glands are located in the cutaneous zone.

11.7 PERANAL SINUSES (ANAL SACS), ANAL AND CIRCUMANAL GLANDS

11.7.1 PARANAL SINUSES OR 'ANAL SACS'

Theses sacs, not to be confused with anal or circumanal glands, are located on both sides of the anal canal between the internal and external anal sphincters. They are found only in carnivores. The sacs may be as small as a pea or as large as a marble. Each sac has an excretory duct about 5mm. long by 2mm. in diameter which opens to the outside at the cutaneous zone (but close to intermediate zones) of the anal canal.

Clinical Considerations:

Anal Sacs: The anal sacs frequently become enlarged due to engorgement of accumulated secretion. They are not infrequently abscessed due to inflammation of the sac and its surrounding tissue.

11.7.2 GLANDS OF THE ANAL CANAL

Circumanal Glands ("Perinanal glands."): These glands of the skin are located in the cutaneous zone of the anal canal around the anus. The glands are of sebaceous and non-sebaceous cell types.

11.7.3 Clinical Considerations:

Circummanal Glands: These glands continue to grow throughout the life of the unaltered male dogs and ademona tumors are common in this region in older dogs.

Anal Glands: These glands are located in the anal sinuses between the anal columns in the columnar zone of the anal canal.

11.8 GLANDS OF THE PARANAL SINUSES

These glands are found in the walls of the paranal sinuses or apocrine, sudoriparous tubules which lie in the walls of the anal sac and in the walls of the excretory ducts. The secretion is a foul-smelling, serous to pasty, brownish grey liquid and is stored in the anal sacs until it passes into the cutaneous zone of the anal canal.

11.9 UREOGENITAL REGION

The urogenital region extends ventrally and cranially from a line connecting the two ischiatic tuberosities to the caudal scrotal area in the male. In the cat the scrotum is included in the urogenital region. From a clinical standpoint, this region is most important in the female and will be discussed only for the female dog.

11.10 MUSCLES OF THE UROGENITAL REGION

11.10.1CONSTRICTOR VESTIBULI

The vulva possesses two striated circular muscles, the most cranial of the two being the strong constrictor vestibuli. Its fibers run in a cranioventral direction encircling the vestibule, urethra and caudal vagina. It blends with the external border of the constrict vestibule.

11.11Functional Anatomy

The constrictor vestibuli constricts the vestibule and during coitus, its effect is to constrict caudal to the bulbus glandis, compressing the dorsal vein of the penis and preventing the male from withdrawal for a period of time (10-30 minutes) following ejaculation.

11.12 CONSTRICTOR VULVAE MUSCLE

The constrictor muscle of the labia is weakly developed in the female dog which blends dorsally with the external anal sphincter muscle and attaches to the coccygeal fascia at the level of the first and second coccygeal vertebrae. It encircles the vulva and vestibule for a distance of about 1 cm. caudal to the area where the urethra enters the vestibule. Its action is to lift the labia dorsally to facilitate intromission of the penis into the vagina.

The constrictor vestibuli and vulvae are homologous to the bulbospogiosus (bulbocavernosus) muscle in the male.

11.13 URETHRALIS, ISCHIOURETHRALIS AND ISCHIOCAVERNOSUS M.

The urethralis, ischiourethralis and ischiocavernosus muscles are small in the female dog. The urethralis encircles the urethra while the ischiourethralis and ischiocavernosus muscles extend from the tuber ischii to the central tendon of the perineum (perineal body) and the crura of the clitoris respectively.