240 



DISSECTION OF THE PERINEUM. 



and nerves 

 in the space. 



First cut in 

 lithotomy- 

 enters this 

 space. 



Muscles of 

 rectum. 



Corrugator 

 cutis ani : 

 attach- 

 ments ; 



Superficial 

 sphincter ; 



origin ; 



insertion ; 



relations ; 



and use. 



Deep 



sphincter, a 

 pale band, 



is part of 

 fibres of 

 intestine ; 



Insertion of 

 levator ani 



half an inch of the margin of the ischial ramus. Crossing the centre 

 of the hollow are the inferior hsemorrhoidal vessels and nerve (b), 

 brandies of the pudic. At the anterior part, for a short distance, 

 two superficial perineal nerves ( 2 ) (of the pudic) ; and at the posterior 

 part is a small branch of the fourth sacral nerve ( 3 ), with cutaneous 

 offsets of the sacral plexus ( 4 ) and inferior haemorrhoidal vessels (c), 

 bending round the gluteus. 



The surgeon sinks his knife into, this space in the first incision 

 in the operation of lateral lithotomy : and as he carries it from 

 before backwards, he will divide the superficial hasmorrhoidal vessel 

 and nerve. 



MUSCLES. Connected with the lower end of the rectum are four 

 muscles, viz., a fine cutaneous muscle, and two sphincters (external 

 and internal), with the levator ani. 



CORRUGATOR CUTIS ANI (fig. 91, B). This thin subcutaneous 

 layer of involuntary muscle surrounds the anus with radiating fibres. 

 Externally it blends with the subdermic tissue outside the internal 

 sphincter ; and internally it enters the anus and ends in the 

 submucous tissue within the sphincter. 



Action. This muscle draws upwards and inverts the mucous 

 membrane of the lower end of the gut, after it has been protruded 

 and everted in the passage of the faeces. 



The EXTERNAL SPHINCTER (sphincter ani externus ; fig. 91 A and 

 fig. 92) is a flat, orbicular muscle, which surrounds the anal open- 

 ing. It arises posteriorly by a fibrous band from the back of the 

 coccyx near the tip, and by fleshy fibres on each side from the sub- 

 cutaneous fatty layer. Its fibres pass forwards to the anus, where 

 they separate to encircle that aperture ; and they are inserted in front 

 into the central point of the perineum, and into the superficial fascia 

 by a fleshy slip on each side. 



The sphincter is close beneath the skin, and partly conceals the 

 levator ani. The outer border projects over the ischio-rectal fossa ; 

 and the inner is contiguous to the internal sphincter. 



Action. The muscle gathers into a roll the skin around the anus, 

 and occludes the anal aperture. Commonly the fibres are in a state 

 of involuntary slight contraction, but they may be firmly contracted 

 under the influence of the will. 



The INTERNAL SPHINCTER (sphincter ani interims ; fig. 91, c) 

 is situate round the extremity of the intestine, internal to the pre- 

 ceding muscle, and its edge will be seen by removing the corrugator 

 muscle and the mucous membrane. The fibres of the muscle are 

 pale, fine in texture, quite separate from the surrounding external 

 sphincter, and encircle the anus in the form of a ring about half an 

 inch in depth. The muscle is a thickened band of the involuntary 

 circular fibres of the large intestine, and is not attached to the bone. 



Action. This sphincter assists the external in closing the anus ; 

 and its contraction is altogether involuntary. 



The LEVATOR ANI (fig. 91, Band fig. 92) can be seen only in part ; 

 and the external sphincter may be detached from the coccyx, in 



