426 MYOLOGY 



the main portion of the muscle, arises from a narrow tendinous band, the anococcy- 

 geal raphe, which stretches from the tip of the coccyx to the posterior margin of 

 the anus; it forms two flattened planes of muscular tissue, which encircle the anus 

 and meet in front to be inserted into the central tendinous point of the perineum, 

 joining with the Transversus perinsei superficialis, the Levator ani, and the Bul- 

 bocavernosus. The deeper portion forms a complete sphincter to the anal canal. 

 Its fibers surround the canal, closely applied to the Sphincter ani internus, and in 

 front blend with the other muscles at the central point of the perineum. In a 

 considerable proportion of cases the fibers decussate in front of the anus, and are 

 continuous with the Trans versi perinsei superficiales. Posteriorly, they are not 

 attached to the coccyx, but are continuous with those of the opposite side behind 

 the anal canal. The upper edge of the muscle is ill-defined, since fibers are given 

 off from it to join the Levator ani. 



Nerve Supply. A branch from the fourth sacral and twigs from the inferior hemorrhoidal 

 branch of the pudendal supply the muscle. 



Actions. The action of this muscle is peculiar. (1) It is, like other muscles, always in a state 

 of tonic contraction, and having no antagonistic muscle it keeps the anal canal and orifice closed. 



(2) It can be put into a condition of greater contraction under the influence of the will, so as 

 more firmly to occlude the anal aperture, in expiratory efforts unconnected with defecation. 



(3) Taking its fixed point at the coccyx, it helps to fix the central point of the perineum, so that 

 the Bulbocavernosus may act from this fixed point. 



The Sphincter ani internus (Internal sphincter ani) is a muscular ring which 

 surrounds about 2.5 cm. of the anal canal; its inferior border is in contact with, 

 but quite separate from, the Sphincter ani externus. It is about 5 mm. thick, and 

 is formed by an aggregation of the involuntary circular fibers of the intestine. 

 Its lower border is about 6 mm. from the orifice of the anus. 



Actions. Its action is entirely involuntary. It helps the Sphincter ani externus to occlude 

 the anal aperture and aids in the expulsion of the feces. 



2. A. The Muscles of the Urogenital Region in the Male (Fig. 406). 



Transversus perinaei superficialis. Ischiocavernosus. 



Bulbocavernosus. Transversus perimei profundus. 



Sphincter urethrse membranacese. 



Superficial Fascia. The superficial fascia of this region consists of two layers, 

 superficial and deep. 



The superficial layer is thick, loose, areolar in texture, and contains in its meshes 

 much adipose tissue, the amount of which varies in different subjects. In front, 

 it is continuous with the dartos tunic of the scrotum; behind, with the subcuta- 

 neous areolar tissue surrounding the anus; and, on either side, with the same fascia 

 on the inner sides of the thighs. In the middle line, it is adherent to the skin on 

 the raphe and to the deep layer of the superficial fascia. 



The deep layer of superficial fascia (fascia of Colles) (Fig. 405) is thin, aponeurotic 

 in structure, and of considerable strength, serving to bind down the muscles of 

 the root of the penis. It is continuous, in front, with the dartos tunic, the deep 

 fascia of the penis, the fascia of the spermatic cord, and Scarpa's fascia upon the 

 anterior wall of the abdomen; on either side it is firmly attached to the margins 

 of the rami of the pubis and ischium, lateral to the crus penis and as far back as 

 the tuberosity of the ischium; posteriorly, it curves around the Transversi perinsei 

 superficiales to join the lower margin of the inferior fascia of the urogenital dia- 

 phragm. In the middle line, it is connected with the superficial fascia and with the 

 median septum of the Bulbocavernosus. This fascia not only covers the muscles 

 in this region, but at its back part sends upward a vertical septum from its deep 

 surface, which separates the posterior portion of the subjacent space into two. 



