1676 



HUMAN ANATOMY. 



part in both sexes, pass by the rectum so as to compress it, although some enter 

 its walls and mingle with those of the sphincters. 



Nerve. — A branch from the sacral plexus (sometimes there are more than one) 

 runs to the levator ani on its upper surface. The fibres come from the third and 

 fourth sacral nerves. According to some, the muscle also receives fibres from the 

 inferior hemorrhoidal branch of the pudic nerve. 



The coccygeus (Fig. 1424), a triangular muscle arising from the spine of the 

 ischium and inserted into the border of the coccyx, is in the same plane and practi- 

 cally continuous with the levator ani. The two muscles of both sides have been well 

 called the diaphragm of the pelvis. They form a funnel-like structure with the walls 

 converging downward to the anal canal, and an anterior opening for the prostate in 

 the male and the vagina and urethra in the female. 



Ftg. T424. 



Obturator 'internus 



Ischial spine 

 Obturator canal 



Cut edge of pelvic 

 fascia, white line 



Pubic bone. 



Triangular ligament, superior layer ^y 

 Triangular ligament, inferior layer ^ 



— Sacrum (cut) 



Pyriformis 



Coccygeus 



iJevatorani 



Recto-coccygeal fibres 

 Levator ani (cut) 



~~ — Rectum 



Urethra Bulb of penis, covered by muscle 

 Muscles of pelvic floor from within ; section passed to left of mid-line. 



Nerve. — The muscle receives branches from the fourth and fifth sacral nerves 

 and perhaps from the first coccygeal. 



The external sphincter ani (Fig. 1423), situated beneath the skin and car- 

 ried up into the puckering at the anus, is a flat oval muscle composed of striated 

 fibres surrounding the end of the rectum. It arises from the tip of the coccyx, 

 from the skin over it, and from a raphe extending from it to the anus. The fibres 

 diverge on either side to enclose the anus, meeting in front of it at the central 

 point of the perineum (page 191 7), where they mingle with other muscles which 

 meet at that point. Some of the inner fibres completely encircle the anus. In 

 the female some fibres decussate with those of the sphincter vaginae. This sphinc- 

 ter is "external" in two senses: it is nearer the outer surface than the internal 

 sphincter and also surrounds it. 



Nerve. — It is supplied by branches of the fourth sacral and of the inferior 

 hemorrhoidal nerve. 



