MUSCLES OF ANUS. 389 



The inner or longest side of the space is very oblique, and is bounded 

 by the levator ani (D), together with the coccygeus muscle posteriorly; 

 but the outer side is vertical, and is formed by the obturator muscle and 

 the fascia covering it. In front it is limited by the triangular ligament 

 (to be afterwards seen) ; and behind are the great sacro-sciatic ligament, 

 and the largest gluteal muscle. Towards the surface it is covered by the 

 teguments, and is overlaid in part by the gluteus E and the sphincter 

 externus A. 



Position of vessels. Along the outer wall lie the pudic vessels and 

 nerve (#), which are contained in a tube of fascia; opposite the ischial 

 tuberosity, they are situate an inch and a half from the surface, but to- 

 wards the front of the space they approach to about half an inch from the 

 edge of the pubic arch. Crossing the centre of the hollow are the inferior 

 hflemorrboidal vessels and nerves (b) branches of the pudic. At the an- 

 terior part, for a short distance, are two superficial perinaeal 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 sciatic vessels (c) and nerve ( 4 ) 

 bending round the gluteus. 



Into this space the surgeon sinks his knife in the first incisions in the 

 lateral operation of lithotomy ; and as he carries it from before backwards, 

 he will divide the superficial haBinorrhoidal vessels and nerve. 



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

 cles, viz., a fine cuticular muscle, and two sphincters (external and inter- 

 nal) with the levator ani. 



Corrugator cutis ani 1 (fig. 129, 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 in- 

 ternally it enters the anus and ends in the submucous tissue within the 

 sphincter. 



Action. By the contraction of the fibres the skin is raised into ridges 

 radiating from the anus, such as may be seen when that aperture is firmly 

 closed. 



The EXTERNAL SPHINCTER (fig. 129, A ) (sphincter ani externus) is a 

 flat, thin, orbicular muscle, which surrounds the lower part of the rectum. 

 The fibres form ellipses around a central aperture, as in other orbicular 

 muscles. 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 subcuta- 

 neous fatty layer. Its fibres pass forwards to the anus, where they sepa- 

 rate to encircle that aperture ; and in front they are inserted into the 

 central point of the perinoeum, and into the superficial fascia by a rather 

 wide fleshy slip on each side. 



The sphincter is close beneath the skin, and conceals partly 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 : by its contraction it makes tense also the cen- 

 tral point of the perineum. 



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 (fig. 129, c ) (sphincter ani internus) is 

 situate around the extremity of the intestine, internal to the preceding 



1 Illustrations of Dissections, p. 243. Lond. 1865. 



