356 Mr. H. G. Seeley on a Theory 
distinguish the more anterior or sensory part of the neural column 
and canal from the part which is always more or less uniform, 
and is called the spinal column; it also exhibits the fact that 
a mouth may exist without having the least connexion with the 
cranium,—thus showing that just as a skull must be a result of 
functional development of the organs of sense at one end of the 
nervous column, so by modification the apparatus around the 
commencement of the digestive canal takes the form of jaws and 
facial bones. Thus, however close the jaws may be brought in 
contact with the cranium, and however the primitive cartilages 
which form the prehensile end of the digestive canal may be 
modified by adaptation to other ossifications, they constitute a 
structure which can only owe its development, like everything 
else, to the higher requirement, or differentiation, of the func- 
tion in which it took its rise; and so, though forming no part 
of the original structure of the cranium in the lowest vertebrata, 
it constitutes by adaptation in higher forms of life an essential 
part of the skull. And, on the other hand, since the cranium 
is sometimes wanting (and in Amphiovus there is nothing which 
can be separated from the spinal cord as a brain), it would be hard 
to regard any brain as more than a functional overgrowth of the 
end of the spinal cord, and therefore to do otherwise than believe 
that its osseous case would be originally formed on the same plan 
with the vertebra, yet speedily and enormously modified by the 
different functions which it subserves. Then, just as the brain, 
from being inseparable from the spinal cord at first, comes 
at last to be a structure as distinct as may be, there is here a 
modification not unlike that which separates the segments of a 
limb (only greater), so that, though both are parts of the same 
organ, their structure and functions are very different. And there- 
fore, although the covering of the brain may in some organisms 
be inseparable from the vertebre, there can only be expected to 
be the same degree of correspondence between the skull and the 
vertebral column that there is between the brain and the spinal 
cord. Ifa brain has parts which have no representatives in the 
spinal cord, it will not be surprising if the brain-case has parts 
which are not found in the case for the spinal cord. 
If a skull is examined, it will be found to be the outlet for, or 
rather the entrance to, the nervous system ; this part is occupied 
by the bram. Secondly, it is the entrance to the digestive sys- 
tem; and this part is constituted by the jaws. And, lastly, it is 
the entrance of the lungs, respiration being carried on through 
the nasal apertures. All these several forces of eating, breathing, 
and observing and thinking exercise great pressure and tension 
on the regions they affect ; and it is precisely these which we have 
already seen ossifying the skeleton. Seeing how the small epi- 
