Part 64

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′).

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. 114, i).--A small bundle of fibres which arises from the medial surface of the ischium, just dorsad of the origin of the ischiocavernosus, and passes mediad to join the sphincter ani internus (c).

=M. caudoanalis= (S.-D.) (Fig. 113, f; Fig. 114, g).--A slender, flat bundle of fibres having origin on the middle line of the ventral surface of the second and third caudal vertebræ. It passes caudoventrad, lying between the levator ani and the caudorectal (Fig. 113, e; Fig. 114, j), and unites with the ventral portion of the sphincter ani internus (Fig. 113, h).

Action.--Draws the anus craniodorsad.

One side of the pelvis has been removed, a, a′, M. extensor caudæ lateralis; b, M. abductor caudæ externus; c, c′, M. flexor caudæ longus; d, M. flexor caudæ brevis; e, M. caudorectalis; f, M. caudoanalis; g, M. caudocavernosus; h, M. sphincter ani internus; i, M. sphincter ani externus; j, M. levator scroti; k, M. rectocavernosus; l, M. bulbocavernosus; m, M. ischiocavernosus (cut); n, M. compressor urethræ membranaceæ. 1, tail; 2, rectum; 3, bulbourethral or Cowper’s gland; 4, prostate gland; 5, symphysis pubis; 6, penis; 7, glans penis; 8, testis; 9, spermatic cord.]

=M. caudorectalis= (Fig. 113, e; Fig. 114, j).

Origin from the ventral surface of the sixth and seventh caudal vertebræ. A small, at first unpaired band two or three millimeters wide is formed, which passes cranioventrad, soon dividing into two lateral halves. These spread out over the sides of the rectum, forming a broad sheet of fibres which pass into the walls of the rectum, among the transverse fibres of the latter. This muscle is covered by the caudoanalis (Fig. 113, f), caudocavernosus (g) (or caudovaginalis, Fig. 114, h), iliocaudalis, and levator ani.

b. Muscles Peculiar to the Male (Fig. 113).--=M. levator scroti= (j).--This is a band of fibres which passes ventrad in the median line from the sphincter ani externus (i) onto the scrotum. Here it spreads out beneath the skin, forming especially a well-marked bundle in the median furrow between the two testes.

=M. rectocavernosus=, or =retractor penis= (k).--A small bundle of fibres which arises in two parts from the ventral surface of the sphincter ani internus (h). The two parts unite into a single bundle which passes caudad on the middle line of the ventral surface of the penis; it is inserted into the corpus cavernosum just proximad of the glans. The muscle is covered only by integument, and overlies the ischiocavernosus (m).

=M. caudocavernosus= (S.-D.) (g).--A slender bundle just craniad of the caudoanalis (f).

Origin on the median ventral line of the first two caudal vertebræ. The muscle passes caudoventrad, lying between the levator ani and the caudorectal (e). It divides into two bands, one of which is inserted into the base of the corpus cavernosum, while the other extends farther caudad and is inserted at the distal extremity of the corpus cavernosum.

Action.--Flexes the penis (bends it backward).

=M. bulbocavernosus= (=accelerator urinæ=) (l).--The two muscles cover the ventral surface of the penis.

Origin of each from a median raphe, which passes from the bulbous portion of the urethra toward the anus. The fibres pass toward the distal end of the penis and have their

Insertion into the distal half of the lateral surface of the corpus cavernosum penis.

=M. compressor urethræ membranaceæ= (n).--A thick layer of striated muscle-fibres which surrounds the urethra between Cowper’s gland (3) and the prostate (4). The fibres have a circular course, and the cranial ones are attached to the crura of the penis. The other fibres have no fixed attachment.

c. Muscles Peculiar to the Female (Fig. 114).--=M. levator vulvæ= (Strauss-Durckheim), or =constrictor cunni= (b).--This is homologous with the levator scroti of the male. It consists of a band of fibres which pass ventrad from the external sphincter ani (a) and surround the vulva (3), lying immediately beneath the integument.

=M. constrictor vestibuli=, or =rectovaginalis= (Strauss-Durckheim) (d).

a, M. sphincter ani externus; b, M. levator vulvæ; c, M. sphincter ani internus; d, M. constrictor vestibuli; e, M. ischiocavernosus (cut); f, M. urethralis; g, M. caudoanalis; h, M. caudovaginalis; i, M. transversus perinei; j, M. caudorectalis. 1, the tail; 2, anus; 3, vulva; 4, rectum; 5, vagina; 6, neck of the bladder.]

Origin from the sides of the sphincter ani internus (c). The muscle forms a distinct bundle two or three millimeters wide, which passes ventrocaudad and is inserted into the ventral surface of the urogenital sinus, caudad of the insertion of the ischiocavernosus (e).

=M. caudovaginalis= (Strauss-Durckheim) (h).--A slender band just craniad of the caudoanalis (g), and corresponding to the caudocavernosus of the male.

Origin from the median line of the ventral surface of the first two caudal vertebræ. The muscle passes caudoventrad, lying between the levator ani and the caudorectal (j), and is inserted into the ventral side of the urogenital sinus, at the base of the clitoris.

=M. urethralis= (f).--This consists of fibres surrounding the cranial part of the urogenital sinus and the caudal parts of the vagina and neck of the bladder.

Origin partly on the caudal part of the symphysis of the ischium, partly from the ventral surface of the urogenital sinus, where the fibres are attached to the corpora cavernosa clitoridis. The fibres pass dorsad over the surface of the sinus, and over the surface of the union of the vagina and neck of the bladder, to be inserted into the sides of the vagina and the dorsal surface of the urogenital sinus.