THE UROGENITAL SYSTEM. 269 



on each side then continue to the scrotum as the levator scroti 

 (Fig. 113,7) or to tne vu l va as the levator vulvae (Fig. 1 14, b}. 

 Fibres also pass onto the anal pouch and unite with the sphinc- 

 ter 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, n). 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 vertebras, close to the muscle of the 

 opposite side. This muscle is frequently continuous with the 

 iliocaudalis (Fig. 162, 11'). 



Action. Bends the tail and compresses the rectum. 



M. ischiocavernosus (Fig. 113, m\ Fig. 114, e). A small, 

 flat, spindle-shaped muscle which lies upon the crus of the penis 

 or clitoris. Each has 



Origin from the caudal border of the ramus of the ischium, 

 about one centimeter from the median line. 



Insertion, in the male, into the whole outer surface of the 

 crus penis, or bulb of the corpus cavernosum penis. In the 

 female the muscle is smaller than in the male, and the insertion 

 is into the ventral surface of the urogenital sinus, at the base 

 of the clitoris. 



M. transversus perinei (Fig. 1 14, z). A small bundle of 

 fibres which arises from the medial surface of the ischium, just 



