THE SKELETON 31 



vertebrae occur. In the adult man the regular number of caudal 

 vertebrae is five, whereas the number may be either four or five 

 in the adult woman. 1 



In the female embryo four such vertebras are found as early 

 as the end of the third month, and the end of the caudal portion 

 of the vertebral column is in the female at all times more liable 

 to variation than in the male. On the other hand, the whole 

 vertebral column of the female is much more constant, with 

 regard to the limits and detailed characters of its separate 

 sections, than that of the male. 



The complete development of the caudal vertebrae is not 

 concluded at birth, for their ossification has not then commenced ; 

 they are in this condition subject to the most varied influences, 

 which may cause further fusion, reduction, or deviation from the 

 sagittal plane (lateral curvature of the terminal vertebrae) (cf. 

 Fig. 24). 



But what defines the human tail ? In answering this ques- 

 tion we cannot do better than follow Keibel, who rightly points 

 out that the definition of the tail in human anatomy must be 

 in strict harmony with that of Comparative Anatomy, and that 

 therefore so much of the vertebral column as is posterior to that 

 (sacrum) which attaches the pelvic girdle is caudal. Since, 

 however, the relation of the limbs to the axial skeleton is of a 

 secondary nature, Comparative Anatomy cannot help us in the 

 important early stages. We can only deal with this difficulty 

 by dividing up the body of the embryo into regions, each con- 

 taining a certain number of segments, and in so doing we cannot 

 avoid ascribing to the regions the number of segments which 

 are found in the adult. In Man, therefore, whom we are now 

 considering, we refer the first seven vertebras to the cervical 

 region, and the twelve which follow to the thoracic ; the lumbar 

 and " sacral " regions each have five, and the remainder belong to 

 the caudal. 



In all Vertebrates, however, a shifting of the pelvic girdle 

 which occurs during embryonic development has to be taken 

 into account ; and in this case the definitions borrowed from 

 the adult are not altogether applicable. His, Fol, and Keibel, 



1 The most reduced vertebral columns are always those of females. Sexual 

 requirements probably account for this, and for the fact that synostotic union of the 

 first coccygeal with the last sacral vertebra is less frequent in females than in males. 

 In the latter, the connection between the cornua sacralia and coccygea may even 

 give rise to a fifth pair of sacral foramina, and in such cases the sacrum appears to 

 consist of six vertebrae. 



