438 The IscJiio- Rectal Fossa 



As the pus drains away the cavity contracts, until ii is represented 

 only by a narrow, thick-walled passage a fistula which, if 'complete,' 

 opens both into the bowel and on to the buttock. Such a fistula has 

 small chance of healing without operation, as at and after defalcation 

 its walls are constantly being dragged asunder towards the anus by 

 the sphincter, and from the anus by the levator. Gas may be driven 

 from the rectum into an internal or a complete fistula, rendering the 

 tissues of that neighbourhood emphysematous. 



The structures divided in the operation for fistula are the 

 integument, the external sphincter ani, and the insertion of the levator 

 ani (both muscles are striated) ; the longitudinal and circular fibres 

 (internal sphincter) of the rectum (non-striated) ; the submucous and 

 mucous coats, and branches of the inferior hcCinorrhoidal vessels and 

 nerves. (See illustration on p. 440.) 



(Fistula near the anus may be due to caries of the spine, p. 211.) 



The Internal pudic vessels and nerve run in a tubular sheath 

 of the obturator fascia at a little more than an inch above the ischial 

 tuberosity, giving off the inferior hsemorrhoidal branches which cross 

 the base of the fossa to reach the neighbourhood of the anus. The 

 artery is accompanied by the dorsal nerve of the penis and the super- 

 ficial perineal branches. 



The levator ani arises from the back of the body of the pubes and 

 from the inner surface of the ischial spine (below the origin of the 

 coccygeus) ; and between these points it arises from the pelvic fascia 

 where the oblique sheets are reflected downwards and inwards to the 

 rectum and bladder, and to the anus (v. p. 363). Between the two 

 levatores the rectum is suspended ; the muscles helping to form the 

 inner wall of the ischio-rectal fossae and to close in the pelvic outlet. 

 The muscle is inserted into the tip of the coccyx, and into the fibrous 

 line leading from it to the anus ; into the side of the third piece of the 

 rectum, between the pale fibres of its longitudinal coat and the striated 

 fibres of the external sphincter ; into the central tendon, and still more 

 anteriorly into the side of the prostate. The most anterior part of 

 the muscle is the levator prostatcc. In the female the vagina passes 

 between the levatores ani. 



Relations. The pelvic surface of the muscle is covered by the 

 recto-vesical fascia, and lies against the rectum and prostate. Be- 

 neath it are the anal fascia, the external sphincter, and the fat of the 

 fossa. Its posterior border lies along the lower edge of the coccygeus. 



Supply. Its vessels are derived from the inferior haemorrhoidal ; 

 its nerves come from the internal pudic and the fourth sacral. 



THE PERINEUM 



The outlet of the pelvis is diamond-shaped, the long axis extending 

 from the pubic symphysis to the tip of the coccyx. Its antero-posterior 



