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 difter from most vertebrae 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, arc epiphyses of the ordinary ribs ; and it will hardly 



