ISCHIO-RECTAL FOSSA 



441 



others terminate in the ano-coccygeal raphe. A few fibre-bundles also inter- 

 digitate across the median line, in front of the rectum Cpubo-coccygeal, fig. 395) 

 and some are inserted into the walls of the rectum. The levator ani and coccygeal 

 muscles of the two sides form a funnel-shaped muscular support for the pelvic 

 viscera (fig. 399). When the abdomino-thoracic diaphragm contracts, as during 

 inspiration, the pressure on the viscera is transmitted to the pelvic diaphragm 

 which resists the pressure and elevates the viscera when the abdomino-thoracic 

 diaphragm relaxes. The levator ani muscle also constricts the rectum and pulls 

 it forward and in the female constricts the vagina from side to side. 



As it passes through the pelvic diaphragm, the rectum for about two and a 

 half centimetres is surrounded by a special external sphincter muscle (figs. 

 393, 394, 397), divisible into three concentric layers as described below. This 

 muscle, especially differentiated from the primitive sphincter of the cloaca, 



Fig. 396. — ^Lateral View of Muscles of the Floor of the Pelvis. 

 (A portion of the ischial and pubic bones sawn away.) 



Aperture for superior 

 gluteal vessels and 

 nerve 



Sacrum 



Piriformis 



Aperture for inferior 

 gluteal and pudic 

 vessels and nerve 

 and sciatic nerve 



Ischial spine 



Coccyx 



Coccygeus 



Hindmost fibres of 



levator ani 



Levator ani 



Sphincter ani 

 externus 



Rectum 



White line (arcus 

 tendineus'l of 

 levator ani 



Pubic attachment 

 of levator ani 



Prostate 



Tendinous centre 

 of perineum 



serves to close the rectum. It is supplemented by a sphincter of smooth muscle 

 which lies immediately beneath the mucous membrane of the anus. It is attached 

 behind to the coccygeus, and in irort to the central tendon of the perineum 

 described below. 



The lateral origin of the levator ani, as above described and as shown in figs. 

 396 and 399, is considerabl}^ above the osseous and muscular margin of the pelvic 

 outlet. The muscles of each side converge toward the post-anal region so that a 

 space is left between the lateral wall of the pelvis, and the levator ani and external 

 sphincter (fig. 399). This space, the ischio-rectal fossa, is filled with fat (fig. 400, 

 401, 402). It is deepened laterally by the lower margin of the gluteus maximus 

 muscle (fig. 400). In the fascial canal (Alcock's canal) in the lateral wall of the 



