COCCYGEAL VERTEBRiE. 17 



Tlie lower end or 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 

 narrowed as it descends. It opens below on the posterior surface of the 

 bone between the sacral cornua where the laminae of the last sacral 

 vertebrte are imperfect. From this canal there pass outwards in the 

 substance of the bone four pairs of intervertebral foramina, closed 

 externally by the lateral masses, but opening on the surface by the 

 anterior and posterior sacral foramina. 



Dijfcrcnces in the xc.rcs. — The sacrum of the female is broader in proportion to 

 its length than that of the male ; it is also flatter, and usually inclines backwards 

 from the du-ection of the lumbar vertebras to a greater extent than in the male. 

 But the curvature varies considerably in different skeletons. 



Tariff ies. — Tlie sacrum not mifrequently consists of six vertebra), and some- 

 times, though rarely, of only four. Occasionally the bodies of the iii"st and second 

 vertebrae are not united, though complete imion has taken place in every other 

 part, or the first vertebra may present on one side the usual sacral fonn, while 

 on the other it has the appearance of a lumbar vertebra, and is not miited to 

 the next (see fig. 21, 11, Z'). a peculiarity connected with the oblique form of pelvis. 

 Instances also occur, in which it presents, on both sides, characters intermediate 

 between those of sacral and lumbar vertebras. The lower end of the sacral canal may 

 be open to a greater extent than usual ; it has even been found open throughout. 



Coccygeal vertebrae, or coccyx. — These are very rudimentary ver- 

 tebra3, commonly four, sometimes five, seldom only three in number. 

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

 sents superiorly on the part corresponding to the body an oval concave 

 surface which articulates with the lower end of the sacrum ; two small 

 processes, termed cornua, which project upwards from its posterior 

 margin, connected by ligaments with the sacral cornua ; and a short 



Fig. 14. — Four Coccygeai, Vertebra:, seen from before. — Froji a Fig. 14 



Male Subject of Middle Age (A. T). ^. 



The upper piece is separate from the second ; the three lower are 

 xmited together in one piece, and separated only by grooves. 1 

 above the middle or body of the first coccygeal vertebra ; 1' below the 

 fourth piece ; 6, the transverse portion ; 7, the superior articulat- 

 iuLT tubercle. 



process, which projects from each side, and usually forms, 



with the inferior lateral angle of the sacrum, a notch for 



the fifth sacral nerve, but in some cases unites with that bone to form a 



fifth sacral foramen. 



The remaining three coccygeal vertebraj are much smaller than the 

 first, and correspond solely to vertebral bodies. When separate, the 

 second piece presents an upper and lower flattened surface. The third 

 and fourth pieces are mere rounded nodules. In middle life, the first 

 piece is usually separate, while the three lower pieces are united into 

 one, the original separation l^eing indicated by transverse grooves. 



In advanced life, the coccygeal vertcbra3, having been previously 

 joined into one bone, become also united to the sacrum. This union 

 occurs at an earlier age and more frequently in the male than in the 

 female, but it is subject to much variation. 



