THE SKELETON 31 
vertebre occur. In the adult man the regular number of caudal 
vertebre is five, whereas the number may be either four or five 
in the adult woman.’ 
In the female embryo four such vertebree 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 hable 
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 vertebre 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 vertebree) (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 vertebre 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, 
* 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 coceygeal 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 vertebre. 
