of the Skull and the Skeleton, 355 



the epiphysis takes its origin. But if it were possible that the 

 tension on the neural arches were ever sufficient to produce an 

 impact, then we might reasonably expect that the neural arch 

 itself, like the centrum, should have epiphyses, as, indeed, appears 

 sometimes to be the case between the zygapophyses. And in 

 fishes, where the head is very large and the connexion with the 

 body powerful, there appears sometimes to be such an epiphysis 

 developed, though it is, as perhaps was to be expected, rather an 

 epiphysis of the skull than of the atlas. Thus we are told, by 

 Mr. Robertson and others, that in the carp, for instance, if the 

 bar of bone which bounds the posterior extremity of the exocci- 

 pitals be traced from above downwards, distinct traces of sutures 

 will be seen between it and the exoccipitals on which it rests ; 

 and following it upwards another suture is found dividing it 

 from the supraoccipitals, so that the bars do not meet above to 

 form a complete arch, the supraoccipitals being prolonged back 

 between these two plates and forming the upper part of. this 

 neural arch, which has no centrum of its own, but rests on the 

 basioccipital. Thus it is seen that epiphyses are not limited to 

 the limb-bones and centra of the vertebrae, but that they may 

 be developed on any bone if it is subjected to the requisite ten- 

 sion and pressure. 



And from these considerations I deduce the following theory 

 of the vertebra — viz., that it consists of a centrum or centre of 

 ossification which normally developes three (or more) pairs of 

 epiphyses, any of which may assume the appearance of separate 

 bones and develope epiphyses themselves. Thus in the majo- 

 rity of animals there are, 1st, one pair of epiphyses at the front 

 and back ends of the centrum ; 2ndly, one pair above, to enclose 

 the neural canal ; and, 3rdly, another pair to enclose the viscera. 

 The upper epiphyses are observed to change their position a 

 little with function, while the lower epiphyses may ascend the 

 centrum and become articulated to, and seemingly developed 

 from, the upper epiphyses; all of them may be absent, and 

 the simple original osseous centre will still be accounted a ver- 

 tebra. But, as we shall hereafter see that the whole skeleton 

 may by this law be accounted for and derived from a single 

 ossification, it would be impossible to admit as a vertebra any 

 structure which varied in plan and function from that which is 

 found in the spinal column. 



With this conception of a vertebra it will now be possible to 

 determine what the skull and spinal column have in common, 

 and how far they difi'er. 



Amphioxus lanceolatus appears to demonstrate that in certain 

 vertebrata, where the vertebrate structure is scarcely assumed, a 

 skull need not exist, and that there may be nothing in structure to 



