chap xvi 1 1. j THE PELVIS. 379 



in an insufficient degree to meet and combine " (Com- 

 mittee of the Clinical Society, London, 1885). 



Sacro-coccygeal tumours. The sacro-coccy- 

 geal region is very often the seat of congenital tumours, 

 some of them of such a shape as to form " human 

 tails ; " and to this part of the pelvis has also been 

 found attached a third lower limb leading to the 

 condition known as " tripodism." 



Parasitic foetuses are also frequently found attached 

 to this segment of the spine. In many of the instances 

 of attached foetuses the two individuals have been joined 

 together at this part of the column. Some of the sacro- 

 coccygeal tumours contain epithelial cysts and fragments 

 of skin, muscle, nerve, bone, cartilage, and mucous mem- 

 brane. These strange masses spring from the anterior 

 part of the coccyx, between it and the rectum. By some 

 they are supposed to arise from Luschka's gland, by 

 others (Sutton) from the structures known to embryo- 

 logists as the post-anal gut and the neurenteric passage. 



The sacro-coccygcal joint may be dislocated 

 or diseased. In either affection great pain is kept up 

 from the frequent movement of the part by the 

 muscles attached to the coccyx (the glutens maximus, 

 coccygeus, levator ani, and sphincter). In the 

 luxation the bone may project into the rectum, and 

 thus give trouble. The joint and the parts about 

 it may be the seat of such severe neuralgia (" coccygo- 

 dynia ") as to require excision of the coccyx, or a free 

 division of the structures that cover it behind. The 

 joint and the fibrous tissue about it are supplied by 

 the following nerves ; the posterior divisions of the 

 second, third, and fourth sacral, and the anterior and 

 posterior divisions of the fifth sacral and coccygeal. 



The thinnest parts of the os innominatum 

 are at the bottom of the acetabulum, and in the hollow 

 of the wing of the ilium. In the latter situation the 

 bone has been successfully trephined for iliac abscess. 



