Chapter 10


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Chapter 10. ANATOMY OF THE PERINEUM (PERINEAL REGION)

The perineum is that part of the body wall that covers the pelvic outlet (caudal pelvic aperture) and surrounds the anal and urogenital canals. Because the perineum is part of the body wall, it has a substantial thickness whose deep and superficial boundaries are not the same.

It can be subdivided into anal and urogenital regions (A region of the body is generally considered to be a surface area without depth; but in the case of the perineal region, e.g., the surface area surrounding the anal canal and urogenital passage, it is considered to be a portion of the trunk.); the latter alone has been called the perineum. The two regions can be subdivided by a horizontal line connecting the vetromedial portions of the two udder coxae. The urogenital region extends ventrally and cranially from this line to the caudal attachment of the udder in the female and to the caudal scrotal area in most males.

10.1 THE ANAL REGION

The anal region includes the area between the first few coccygeal vertebrae and the anal canal and areas lateral to the anal canal between that and the ischial tuberosities.

The anal region is continuous with the urogenital region at an approximate transverse line passing below the anal canal and thus between it and the vagina in the female and the urethra in the male (In the cat, the scrotum is included in the urogenital region.).

10.2 PELVIC DIAPHRAGM AND ISCHIORECTAL FOSSA

The internal boundary of the anal region which separates it from the pelvic cavity is the pelvic diaphragm. The pelvic diaphragm is composed of two skeletal muscles and two layers of fascia. They are: levator ani and coccygenus muscles; and external and internal fasciae of the pelvic diaphragm.

The ischiorectal fossa is a deep depression which lies chiefly lateral to the anal canal and bounded laterally by the sacrotuberous ligament. It is pyramidal in shape with its base most superficial and its apex deeply located in either side of the anal canal. It contains a fair amount of fat in animals in good nutritional state.

The internal pudendal artery and vein and the pudendal nerve can be found in the fossa.

10.3 MUSCLES OF THE PELVIC DIAPHRAGM

10.3.1 COCCYGEUS MUSCLE (Formerly called "lateral coccygeus" muscle.) The coccygeus muscle is the most lateral of the two muscles making up the pelvic diaphragm. It is a fan-shaped muscle which extends from the ischial spine of the os coxae, upward and medially to attach to the transverse processes of the first 3-5 coccygeal vertebrae.

10.3.2 LEVATOR ANI MUSCLE (Formerly called "medial coccygeus" in the small animal species, and "retractor ani" muscle in the large animal species.)

The levator ani muscle lies medial and both cranial and caudal to the coccygeus muscle. It is a broad flat, triangular muscle which attaches the medial surfaces of the length of the pelvic symphysis, and portions of the pubis and ilium. The levator ani consists of two parts: an ischial part and an iliopubic portion with the obstructor nerve passing between them. The muscle extends upward to the root of the tail and attaches on the hemal processes of the seventh coccygeal vertebra by a strong tendinous band.

Clinical Considerations :

Perineal Hernia in the Dog

This condition results from a failure of the supporting structures of the pelvic outlet (pelvic diaphragm) to contain the pelvic organs, and is recognized chiefly in older male dogs.

The herniation, which is composed chiefly of retroperitoneal contents, fat principally, but also prostate gland, colon, and urinary bladder, have all been found in the hernial sac.

The levator ani and coccygeus muscles are attached to the external anal sphincter by deep (perineal) fascia and when perineal hernia develops, a protrusion occurs through the fascial cases, through the muscle fibers of the levator ani itself. With loss of support to the rectum in this region by the pelvic diaphragm, there is dilation of the rectum with pouching and sacculation of it between the two above muscles. Feces may accumulate in the sacculation which further complicates the hernia and adds to the tenesmus (straining) which accompanies the condition.

It has been stated that estrogen secretion by the aging testicle may be a cause of relaxation of the pelvic diaphragm and for this reason, castration is often recommended after surgical repair in order to prevent a reoccurrence.