OF THE VOMEE, ETHMOID, MND INTEEMAXILLAEY BONES. 
307 
right and left portion, exactly as the vomer, by a prolongation downwards, divides the 
anterior haemal arches. 
4. The intermaxillaries, maxillaries, and nasals are the osseous elements of one seg- 
ment. Of this we might find sufficient evidence in their relations in mammals ; and 
indeed the proposition does not stand in need of much proof, if it be once considered 
certain that the vomer, lateral masses of the ethmoid, and palatals belong to one 
segment, and that the mesial-palatine processes of the intermaxillaries are the centrum 
of the segment following. But that we may fully understand the parts played by the 
intermaxillaries, maxillaries, and nasals respectively, we must defer our remarks on this 
subject till we have examined the arrangements in other classes. 
We shall now glance for a single moment at the manner in which this explanation of 
the anterior segments of the skull affects the general view of the cranial segmentation. 
That there is an occipital segment, and that the postsphenoid and parietals are por- 
tions of another segment, is generally admitted ; and we have concluded that the frontal 
and central plate of the ethmoid are elements of a third segment. These then are the 
three segments which roof-in the cranial cavity. They are all complete above. Their 
centra are successively smaller in their order from behind ; and the ffoor of the neural 
arches which they form gradually turns upwards as it passes forwards. 
In front of the occipital segment lie the petrous bones *, and connected with them are 
the organs of hearing. In front of the parietal segment lies the presphenoid, connected 
with which are the organs of \ision. In front of the frontal segment lies the ethmo- 
vomerine segment, and connected with it are the organs of smell. None of the three 
segments connected with the special senses has a complete neural arch. With the most 
posterior of them no centrum is connected ; the second (the presphenoid) has a small 
centrum ; and the most anterior has a larger centrum — the vomer : and the tendency of 
these centra, contrary to the tendency of the centra of the brain-protecting segments, is 
to curve downwards as they pass forwards. Thus there are two alternating sets of sclero- 
tomes, which may be distinguished as the protective and the sensory; while foremost 
of all is an imperfect and peculiar seventh and terminal sclerotome — the facial, which 
may be considered as binding these two sets together. On this seven-segmented plan I 
believe the head to be formed in all vertebrata ; and although it be true that in certain 
cases there are no sclerous elements of the cranium developed in connexion with either 
the ear or the eye, these organs are nevertheless themselves portions of segments lying 
between those in connexion with which the neighbouring protective sclerotomes are 
developed. The upward tendency of the protective, and the downward tendency of the 
sensory sclerotomes, is not seen at all in the fish, but becomes more and more observable 
as we ascend to reptiles, birds, and mammals ; and is exhibited best of all in man, in 
whose structure the idea of vertebrate creation is completed (fig. 27), 
These few and imperfect remarks have been necessitated by the consideration that no 
* It is unnecessary for tlie purpose of this argument to discuss the positions of the interparietal, mastoid, 
or squamous. 
2 T 2 
