508 ABDOMEN 



investigation of the pelvic spinal nerve plexuses, and the sym- 

 pathetic trunk, and the coccygeal glomus (see p. 461). After 

 he has satisfied himself regarding the formation and distribution 

 of the plexuses, and the position and connections of the sympathetic 

 trunk, he should study the pelvic diaphragm. 



The sacral, the pudendal, and the coccygeal nerve-plexus* 

 are the same in both sexes, and the description of their coi 

 stitution and relations will be found on pp. 461 and 462. 



The pelvic portions of the sympathetic trunks an 

 described on p. 464. 



The position and constitution of the coccygeal glomus 

 are given on p. 465. 



THE PELVIC DIAPHRAGM. 



The pelvic diaphragm is described on pp. 459, 460. The 

 dissector should note that the levator ani muscle passes 

 downwards and backwards and that it gives support to the 

 lateral border of the vagina; he should note also that the 

 anterior borders of the two levatores ani muscles embrace 

 the lower part of the vagina very closely, and exert a 

 sphincter action upon it. 



Dissection. After he has studied the pelvic diaphragm the 

 dissector should remove the levator ani to display the obturator 

 internus, and examine the attachment and arrangement of that 

 muscle (see p. 468). He should then study the piriformis (see 

 p. 468), and should complete his dissection of the pelvis by an 

 examination of the pelvic articulations. 



The Pelvic Articulations. The pelvic joints are described 

 at p. 469. In the later months of pregnancy the ligamentous 

 structures of the various pelvic joints become softened and 

 thickened by the infiltration of fluid. The pelvic bones are 

 thus separated from each other to some extent, and the 

 calibre of the pelvic canal is increased, in preparation for the 

 passage of the child. 



