THE UROGENITAL SYSTEM. 269 
on each side then continue to the scrotum as the levator scroti 
(Fig. 113, 7) or to the vulva as the levator vulve (Fig. 114, 4). 
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, 4; Fig. 114, ¢).— 
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 vertebrz, 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, ¢).—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. 114, 7).—A small bundle of 
fibres which arises from the medial surface of the ischium, just 
