of the Skull and the Skeleton. 357 
physial elements of the neck in Plesiosaurus were observed to put 
on an enormous and complex development under the increasing 
pressure of the viscera in the thorax, I cannot but point out 
that the brain presents to the spinal cord precisely the same 
sort of relation which the viscera of the thorax do to those of 
the neck, and therefore to anticipate that the formation of the 
cranium will follow an analogous law. And it has already been 
seen how, under the action of the lungs, &c. the ribs elongated 
and formed epiphyses ; and therefore when this force used in 
breathing comes to be narrowed to a small aperture it accounts 
for the often osseous condition of the trachea, and, coming in 
contact with other ossifications, could hardly fail to develope epi- 
physes: and accordingly we shall see that the nares are gene- 
rally surrounded by the same set of bones, quite regardless of 
the place where they open in the skull, whether at the tip of the 
jaws or near to the brain. And, finally, it would be superfluous 
to insist on the force manifested in using the jaws; and thus we 
shall see that the degree of development in the maxillary and 
premaxillary bones will be entirely proportionate to the pressure 
and tension allowed by the presence or absence of teeth, and 
the mode in which the jaws are used. 
If an ossified brain-case is examined, it will be seen to be 
more or less easily divisible into three segments, as, indeed, is 
generally admitted. - The first of these, following Professor 
Huxley, I take to consist of the basioccipital, the exoccipitals, 
and the supraoccipital; the second consists of basisphenoid, 
the alisphenoids, and the parietals; while the third is made up 
of the presphenoid, orbitosphenoids, and frontals. 
As compared with vertebree, it will be seen, as is remarked by 
Mr. Robertson and others, that these segments differ in being 
roofed in by bones (the supraoccipital, parietals, and frontals) to 
which there is obviously nothing corresponding in the covering 
of the spinal cord; and they also differ from most vertebre in 
the arches touching each other at every point. 
Thus, remembering that the brain was originally but the 
anterior end of the spinal cord, and so far, as evidenced by the 
law of pressure and tension which has been considered, must 
have been roofed in by similar structures, we find that when the 
brain expands in height and size above the proportions of the 
spinal cord, it becomes roofed in by additional bones, just 
as the thorax was when it expanded in depth below the limits 
of the small neck. So that the alisphenoids are epiphyses of 
the basisphenoid, just as the neurapophyses are epiphyses of 
an ordinary centrum, and the parietals are epiphyses of the ali- 
sphenoids, just as the sternal ribs or sternum in birds, for 
instance, are epiphyses of the ordinary ribs; and it will hardly 
