ABDOMEN 



Dissection. Clean the sphincter ani externus. Make an 

 incision through the fascia which covers the muscle, from the 

 tip of the coccyx to the anus, carry it forwards round the sides of 

 the stitches which were used to close the anus, and then onwards 

 to the central point of the perineum in the median plane. Reflect 

 the fascia to each side and, as the borders of the muscle are 

 defined, secure the branches of the inferior haemorrhoidal nerve 

 and artery and the perineal branch of the fourth sacral nerve, 

 which supply the muscle. 



M. Sphincter Ani Externus. When the external sphincter 



muscle of the anus is 

 cleaned it will be seen 

 to consist of a thick 

 ring of muscular fibres 

 which surround the 

 orifice of the anal canal. 

 Posteriorly, it is attached, 

 by a pointed tendon, to 

 the tip and posterior 

 im'ernu u s rat r surface of th e terminal 



Vesicula P art f tn C COCCyX ' CLH- 



-seminaiis teriorlv, it blends with 



-..._ M. levator am 



Pudendai other penneal muscles 

 nerve and in the central point of 

 Rectum the perineum. As the 

 fibres pass between the 



FIG, 7 8._Diagram. The arrow is directed tW ^* <* attach- 



upwards into the ischio-rectal fossa. The BlCnt, they encircle the 



parietal pelvic fascia is seen upon the anal Orifice and form a 



medial surface of the obturator internus. true o n hinrfpr rrmsrlp 



Observe also the inferior fascia of the >pnmcU 



pelvic diaphragm clothing the lateral Some of the Superficial 



surface of the levator ani and the rectal fibres, both in front of 



fascia upon its medial surface. and behind the ana] 



opening, are directly attached to the skin. It draws its 

 nervous supply from two sources, viz., the fourth sacral nerve 

 and the inferior hcemorrhoidal nerve. 



Contents of the Anal Triangle. The next step in the 

 dissection of the perineum is the display of the ischio-rectal 

 fossa and their contents, but before the dissection is com- 

 menced the dissectors should have a general idea of the 

 position and contents of the fossae. 



Passing through the middle of the anal triangle of the 

 perineum is the anal canal, which terminates at the anus. 



