374 



THE ABDOMEN AND PELVIS 



they may rupture into the anal canal,, in which case an internal 

 sinus is formed. The orifice of an internal sinus is generally 

 found at the pectinate line (p. 370), i.e. the track of the sinus 

 lies below the level of the levator ani and longitudinal muscular 

 coat of the gut. An ischio-rectal abscess may rupture in both 

 directions and so give rise to a complete fistula. As the whole 

 fistula lies below the internal sphincter, it is only necessary to 

 divide the external sphincter, the movements of which tend to 

 keep the fistula open. This should be carried out radially from 

 the anal margin, i.e. at right angles to its fibres, and in one place 



Bladder 



Recto-vesi 

 fascia 



Anal canal 



Obturator internus 



Levator ani, clothed on 



medial side by rectal 



fascia, and on lateral side 



by lower fascia of pelvic 



diaphragm 



Alcock's canal, con- --" 

 taining pudendal 

 vessels and nerve 



FiG. 114. Diagram of the Endo-pelvic Fascia. The pelvis is divided in a 

 frontal plane and the pelvic fascia is represented in red. 



only, otherwise the muscle may fail to unite. In the examination 

 of a case of fistula in ano, it is advisable to pass a probe into 

 the fistula before inserting the finger into the anal canal, as 

 this process may induce spasm of the external sphincter and 

 so make the passage of a probe a matter of greater difficulty. 

 Sometimes the mucous membrane is undermined for some 

 distance above the internal opening of the fistula, and the 

 point of the probe may be felt separated from the finger by 

 the mucous coat alone. 



Abscesses arising above the levator ani, either in connection 

 with the pelvic organs or diseased bone, sometimes pierce the 

 muscle and enter the ischio-rectal fossa from above. 



The Pudendal Nerve arises from the sacral plexus (S. 2, 3, 



