Muscles of the Urogenital Organs, Rectum, and Anus
([Figs. 113] and [114]).—The muscles connected with the caudal openings of the alimentary canal and of the urogenital organs are closely interrelated, a single muscle sometimes acting on parts of both systems. For this reason all these muscles are described together.
The region lying between the anus and the external opening of the urogenital organs is known as the perineum. The perineum is formed chiefly by muscles and fascia.
a. Muscles common to the Male and Female.
—M. sphincter ani externus ([Fig. 113], i; [Fig. 114], a).—This muscle is confounded with the levator scroti ([Fig. 113], j) or the levator vulvæ ([Fig. 114], b). The two take origin in common from the integument on the dorsum of the root of the tail dorsad of the fifth caudal vertebra. There the fibres from the opposite sides are intermingled. The common muscle passes ventrad about the tail as a flat band close to the integument. Dorsad of the anus the inner fibres of the muscles of the opposite sides are united. They then separate and surround the anus as a band five millimeters wide situated beneath the integument. Ventrad of the anus the fibres are again intermingled. Some fibres on each side then continue to the scrotum as the levator scroti ([Fig. 113], j) or to the vulva as the levator vulvæ ([Fig. 114], b). Fibres also pass onto the anal pouch and unite with the sphincter ani internus, forming the constrictors of the anal pouch (Strauss-Durckheim).
M. sphincter ani internus ([Fig. 113], h; [Fig. 114], c).—The sphincter ani internus is a broad and thick band of striated muscle-fibres which surrounds the rectum at the anus. Dorsad the band is about two centimeters broad, while ventrad it is less than one. In the ventral median line some of the fibres pass craniad to help in forming the bulbocavernosus muscle. The muscle surrounds the anal sac.
(The muscle here described under this name is that described under the same name in the cat by Strauss-Durckheim and Mivart; it corresponds, however, to a part of the sphincter ani externus of the dog, as described by Ellenberger and Baum.)
M. levator ani (or pubiocaudalis) ([Fig. 162], 11).—This muscle lies in the pelvic cavity. Each muscle forms a nearly vertical sheet, and between the two are the rectum and the urethra.
Origin from the symphysis of the pelvis.
Insertion into the midventral line of the centra of the third, fourth, and fifth caudal vertebræ, close to the muscle of the opposite side. This muscle is frequently continuous with the iliocaudalis ([Fig. 162], 11′).