12 
or, in other words, one ossification may develope another if suffi- 
cient pressure and tension can be applied to its surface: a law 
which appeared to be as true for the entire animal, as for a 
single bone. 
_ And hence the vertebra was supposed to consist of a centrum 
or centre of ossification which typically developes three pairs 
of epiphyses: one pair, in front and behind, being the epiphyses 
commonly so called; another pair above to enclose the neural 
cord, called neural-epiphyses; and a third pair beneath to en- 
close the viscera, called hemal epiphyses. The author then 
discussed the extent to which these epiphyses actually do pro- 
duce other epiphyses. 
And thirdly, the comparative osteology of the skull, and 
the morphological and embryonic development of the brain were 
considered; and the conclusion drawn, that since the skull 
was the entrance to three distinct regions of the body, the ner- 
vous system, the respiratory system, and the digestive system, it 
must be considered in the relation of its three several regions to 
the corresponding parts of the organism before it can be com- 
pared with the body as a whole. 
The author then considered the ossification of the trachea, 
and showed how by the laws which had been arrived at, ossifica- 
tion would be greater at its termination where it is in contact 
with other bones; and so the circles of bones which surround the 
anterior termination of the respiratory region in the skull were 
regarded as only the modified end of the trachea, and therefore 
could not correspond with any part of an ordinary vertebra. 
The brain-case was recognized as consisting of three seg- 
ments; and such that the basisphenoid corresponded to an 
ordinary vertebral centrum, and developed the presphenoid and 
basi-occipital for its epiphyses in one direction, and the alisphen- 
oids in another; and by a law previously determined it was 
considered that the whole occipital segment was the posterior 
epiphysis of the whole parietal segment, while the whole frontal 
