12 VERTEBRAL COLUMX. 



surface which articulates \vith the body of the last lumbar vertebra, and 

 behind this, a triangular aperture, which leads into the sacral canal, and 

 is bounded behind by the sharp depressed border of the laminae of the 

 first sacral vertebra. On each side of the aperture is an articulating pro- 

 cess with a vertical articular surface looking backwards and inwards like 

 the superior articulating processes of the lumbar vei tebrse. In front of this 

 is a groove which forms part of the last lumbar intervertebral foramen. 

 The external portion of the base presents posterioily an eminence cor- 

 responding to the lumbar transverse process, and in front of that a large 

 smooth convex surface, inclined forwards and continuous with the iliac fosba 

 of the pelvis. 



The apex, formed by the small inferior surface of the body of the fifth 

 sacral vertebra, is transversely oval, and articulates with the coccyx. 



The sacral canal is three-sided, curved with the bone, and gradually nar- 

 rowed as it descends. It opens below on the posterior surface of the bone, 

 between the saural cornua where the laminse of the last sacral vertebra do 

 not unite. From this canal there pass outwards in the substance of the 

 bone four pair's of inter vei tebral foramina, or short canals, each of which 

 divides externally into an anterior and posterior sacral foramen. 



Curvature. The curve of the sacrum varies greatly in different skeletons : in some 

 it is slight and confined to the lower end, while in others it is observable in the whole 

 length, but especially about the middle. 



Differences in the sexes. The sacrum of the female body is broader in proportion 

 to its length than that of the male, so as to approach the form of an equilateral 

 triangle, and is seldom curved greatly, or in its whole extent. The sacium of the 

 female also usually inclines backwards from the direction of the lumbar vertebrse to a 

 greater extent than that of the male. 



Varieties. The sacrum is subject to numerous and considerable variations. It not 

 unfrequently consists of six pieces, and it has been found, but much more rarely, 

 reduced to four. (Soemmerring, " Lehre von den Knochen urid Biindern, &c., 

 herausgegeben von Kudolph Wagner," 1839, p. 128.) It is not fully determined 

 whether these differences depend on increased development of the first coccygeal 

 vertebra and on diminished development of the fifth sacral, or whether there may 

 not be an interposition of a new vertebra in one instance, and the loss of one of the 

 ordinary series in the other. Occasionally the bodies of the first and second sacral 

 vertebrae are not joined, although complete union has taken place in every other 

 part ; and in like manner sometimes a space is left between their arches. The lower 

 end of the sacral canal may be open to a greater extent than usual, in consequence 

 of some of the vertebral laminae not having reached the middle line posteriorly : 

 it has even been found open in its whole extent. (Thomson.) Instances occur in 

 which the first sacral vertebra has on one side the usual sacral form, while on the 

 other it has the form of a lumbar vertebra, and occasionally it presents characters on 

 both sides, intermediate between those of the sacrum and lumbar vertebrse. 



THE COCCYX. 



The coccyx consists most commonly of four rudimentary vertebrse ; some- 

 times of five, seldom of only three. The coccygeal vertebrae diminish 

 gradually in size from above downwards, and are placed in a continuous 

 line with the lower part of the sacrum. 



The first of the series is considerably broader than the others. It presents 

 superiorly a small oval concave surface which articulates with the apex of 

 the sacrum, two lateral projections corresponding with those of the last 

 sacral vertebra, and two bmall processes, termed cornua, corresponding to 

 a certain extent with articulating processes, which project upwards from its 

 posterior aspect and rest upon the sacrul cornua. The lateral parts project 



