16 



THE VERTEBRAL COLUMN. 



and in the Andaman ese 94. In the European female it is about 116. The anthropoid apes 

 have a sacral index varying from 87 in the orang to 72 in the gorilla. (Turner, Joum. Anat., xx, 

 and " Challenger " Reports, Zoology, xvi.) 



Varieties. The sacrum not unfrequently consists of six pieces, a condition which is 

 generally due to the inclusion of the first coccygeal vertebra. More rarely there are only four 

 sacral vertebras. Occasionally the bodies of the first and second vertebras are not united, 

 though complete union has taken place in every other part ; or the first vertebra may present 

 on one side the usual sacral form, while on the other it has the form of a lumbar vertebra, and 

 is not united to the next (see fig. 23, 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 sacral canal may be open below to a 

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



Coccygeal vertebrae, coccyx. These are very rudimentary vertebras, 

 commonly four, sometimes five, seldom only three in number. The first of the 

 series is considerably broader than the others. It presents superiorly, on the part 

 corresponding to the body, an oval concave surface, which articulates with the lower 

 end of the sacrum. From its posterior surface two small processes, termed cornua of 



Fig. 16. THE COCCYX: A, FROM BEHIND; B, FROM BEFORE. (Drawn by D. Gunn.) f 



the coccyx, project upwards ; they represent the pedicles and superior articular 

 processes of the vertebras generally, and are connected at their extremities to the 

 sacral cornua, with which they enclose an aperture the last intervertebral foramen 

 (fig. 14) for the passage of the fifth sacral nerve. On each side the short transverse 

 process projects, and usually bounds, with the lowest part of the lateral margin of the 

 sacrum, a notch for the anterior division of the same nerve ; but in some cases it is 

 united by bone to the lower lateral angle of the sacrum, so as to form a fifth anterior 

 sacral foramen. 



The remaining three coccygeal vertebras are much smaller than the first. 

 The second piece, when separate, has upper and lower flattened surfaces for 

 articulation with the vertebra above and below ; on each side is a rudiment of the 

 transverse process in the form of a slight tubercle ; and on the posterior aspect there 

 may sometimes be seen two small eminences in series with the cornua of the 

 first piece, and representing the last traces of a neural arch. The third and fourth 

 pieces are mere rounded nodules, slightly compressed from above downwards, and 

 corresponding solely to vertebral bodies. In middle life the first piece is usually 

 separate, while the three lower pieces are united into one, the original separation 

 being indicated by transverse grooves. 



In advanced life the coccygeal vertebras, having been previously joined into one 

 bone, may 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. The first piece often joins the sacrum before the union of the rest of the 

 bone. 



