OR VERTEBRA. 39 



back part or base of the conical skull, and which immedi- 

 ately precede and join those of the trunk; we thus obtain 

 a " segment" or " vertebra" of the skull. If we next pro- 

 ceed to separate a little the bones composing this segment, 

 we find those that were most closely interlocked to be in 

 number and arrangement as follows : Two single and 

 symmetrical bones, and two pairs of unsymmetrical bones, 

 forming a circle ; or, if the lower symmetrical bone, which 

 is the largest, be regarded as the base, the other five form 

 an arch supported by it, of which the npper symmetrical 

 bone is the keystone.^ This answers to the "neural" 

 arch of the typical vertebra : the base-bone is the " cen- 

 trum," c; the pair of bones, which articulated with its 

 upper surface and protected the hind division of the brain, 

 form the " neurapophyses," n ; the smaller pair of bones, 

 projecting outwards, like transverse processes, are the 

 " diapophyses," d; the symmetrical bone completing the 

 arch, and terminating above in a long crest or spine, is 

 the "neural spine," iis. It will be observed that the cen- 

 trum is concave at that surface which articulates with the 

 centrum of the first vertebra of the trunk ; the opposite 

 surface is also concave, but expanded and very irregular, 

 in order to effect a much firmer union with the centrum 

 of the next cranial segment in advance — great strength 

 and fixity being required in this part of the skeleton, 

 instead of the mobility and elasticity which is needed in 

 the vertebral column of the trunk. It may be also ob- 

 served that the " neurapophyses" are perforated, like most 

 of those in the trunk, for the passage of nerves ; that the 

 diapophyses give attachment to the bones which form the 

 great inferior or haemal arch ; and that the neural spine 



' See my work "On the Archetype of the Skeleton," 8vo. 1848, p. 10, 

 Fig. 1. 



