34 SURGICAL APPLIED ANATOMY. [Chap. xvm. 



the remarkable double-wedge-shaped outline of the 

 bone, and by the very dense ligaments that bind it in 

 its place. The thick end of the main wedge of the 

 sacrum is in front, and therefore the strongest 

 ligaments are to be found behind the bone, as if to 

 prevent it from slipping forwards, or from becoming 

 rotated forwards on its transverse axis. The laminae 

 and spinous processes of the upper part of the sacrum 

 may be absent from congenital defect. A like 

 deficiency will probably be observed in the lower 

 lumbar vertebrae in such cases, and through the 

 median gap so formed the spinal membranes may 

 protrude, forming a large cystic tumour, known as 

 spina bifida. The sacro-coccygeal 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 "tri- 

 podism." 



The sacro-coccygeal 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 gluteus 

 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. 



