OF THE PERINEUM AXD PELVIC OUTLET. 



459 



fibre, which surround the margin of the anus, and join in a commissure in front 

 and behind. 



Nerve-supply. A branch from the anterior division of the fourth sacral and 

 the inferior haemorrhoidal branch of the internal pudic. 



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 orifice closed. 2. It can be put into a condition of greater contraction 

 under the influence of the will, so as to more firmly occlude the anal aperture. 

 3. Taking its fixed point at the coccyx, it helps to fix the central point of the 

 perinaeum, so that the Accelerator may act from this fixed point. 



The Internal sphincter is a muscular ring which surrounds the lower extremity 

 of the rectum for about an inch, its inferior border being contiguous to, but quite 

 separate from, the External sphincter. This muscle is about two lines in thickness, 

 and is formed by an aggregation of the involuntary circular fibres of the intestine. 

 It is paler in color and less coarse in texture than the External sphincter. 



Actions. Its action is entirely involuntary. It helps the External sphincter 

 to occlude the anal aperture. 



The Levator ani (Fig. 296) is a broad, thin muscle, situated on each side of the 

 pelvis. It is attached to the inner surface of the sides of the true pelvis, and 



Ureter 



Eecto-tesical 

 fascia, 



Peltic fasci 



Veins of 

 prostatic plexu* 



Trans, perinfi 

 muscle 



FIG. 2%. Pelvis seen from behind, showi 



ani. (Henlej 



descending unites with its fellow of the opposite side to form the floor of the pelvic 

 cavity. It supports the viscera in this cavity and surrounds the various structures 

 which pass through it. It arises, in front, from the posterior surface of the body 

 and ramus of the os pubis on the outer side of the symphysis ; posteriorly, from 

 the inner surface of the spine of the ischium ; and between these two points from 

 the angle of division between the obturator and recto-vesical layers of the pelvic 

 fascia at their under part. The fibres pass downward to the middle line of the floor 

 of the pelvis, and are inserted, the most posterior into the sides of the apex 

 of the coccyx ; those placed more anteriorly unite with the muscles of the opposite 

 side, in a median fibrous raphe, which extends between the coccyx and the margin 

 of the anus. The middle fibres, which form the larger portion of the muscle, are 



