454 THE MU8CLES AND FASCIAE 



by its apex from the spine of the ischium and lesser sacrosciatic ligament, and 

 inserted by its base into the margin of the coccyx and into the side of the lower piece 

 of the sacrum. It assists the Levator ani and Pyriformis in closing in the back 

 part of the outlet of the pelvis. 



Nerve-supply. The Coccygeus is supplied by a branch from the fourth and fifth sacral 

 nerves. 



Actions. The Levatores ani constrict the lower end of the rectum and vagina. They elevate 

 and invert the lower end of the rectum after it has been protruded and everted during the expul- 

 sion of the feces. The Coccygei muscles pull forward and support the coccyx, after it has been 

 pressed backward during defecation or parturition. The Levatores ani and Coccygei together 

 form a muscular diaphragm which supports the pelvic viscera. 



Muscles of the Ischiorectal Region. 



Corrugator cutis ani. Internal sphincter ani. 



External sphincter ani. Levator ani (described on page 451). 



Coccygeus (described on page 453). 



The Corrugator Cutis Ani. Around the anus is a thin stratum of involun- 

 tary muscle fibre which radiates from the orifice. Internally, the fibres fade 

 into the submucous tissue, while externally they blend with the true skin. By 

 its contraction it raises the skin into ridges around the margin of the anus. 



The External sphincter ani (m. sphincter ani externus) (Figs. 334, 335, 338, and 

 344) is a thin, flat plane of muscle fibre, elliptical in shape and intimately adher- 

 ent to the integument surrounding the margin of the anus. It measures about 

 three or four inches in length from its anterior to its posterior extremity, being 

 about an inch in breadth opposite the anus. It consists of two strata, super- 

 ficial and deep. The superficial, constituting the main portion of the muscle, 

 arises from a narrow tendinous band, the anococcygeal raphe, which stretches 

 from the tip of the coccyx to the posterior margin of the anus ; it forms two flattened 

 planes of muscle tissue, which encircle the anus and meet in front of be inserted 

 into the central tendinous point of the perineum, joining with the Transversus 

 perinei, the Levator ani, and the Accelerator urinae. The deeper portion forms 

 a complete sphincter to the anal canal. Its fibres surround the canal, closely 

 applied to the Internal sphincter, and in front blend with the other muscles at the 

 central point of the perineum. In a considerable proportion of cases the fibres 

 decussate in front of the anus, and are continuous with the Transversus perinei. 

 Posteriorly, they are not attached to the coccyx, the fibres of opposite sides being 

 continuous behind the anal canal. The upper edge of the muscle is ill-defined, 

 since fibres 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 internal pudic 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 Accelerator urinae may act from this fixed point. 



The Internal sphincter ani (m. sphincter ani internus) is a ring of musch 

 which surrounds the anal canal for about an inch; its inferior border being 

 contiguous with, but quite separate from, the External sphincter. This muscle 

 is about a sixth of an inch in thickness, and is formed by an aggregation of the 

 involuntary circular fibres of the intestine. It surrounds the canal for about an 



