ANATOMY OF VERTEBRATES. 



91 



74 



Hyobrancliial frame, Rana paradoxa. cxxxix. 



The scapular arch, fig. 42, so, 51, retrogrades, like the hyoid, from 

 its primitive position in the larva. 



Cuvier, at thexonclusion of his description of the batrachian skull, 

 remarks, ' This skull does 

 not accord with the theory 

 of the three, four, or seven 

 vertebrae, or even of one 

 (cranial) vertebra, any more 

 than it does with that of the 

 identity in the number of 

 bones ' (in different animals). l 



At the same time he de- 

 termines the special homo- 

 logy of the twenty-six bones, 

 exclusive of the mandible and hyoid apparatus, and assigns to them 

 the same names, --and as regards the majority, correctly, which 

 those bones bear in the rest of the vertebrate province. We have 

 been led, therefore, to look for some higher law within which that 

 of the special conformity may be included. 



In many instances of trunk-vertebra?, the neurapophyses meet 

 below, as well as above the neural axis, their bases being extended 

 towards each other so as to interpose between that axis and the 

 vertebral centrum. This condition is repeated by the exoccipitals 

 which form the neural arch of the epencephalon, and encompass it, 

 in Batrachia, giving passage to its chief pair of nerves and de- 

 veloping articular processes for the succeeding vertebra. The two 

 pairs of neurapophyses in advance, retain the more ordinary rela- 

 tions of these elements, the more expanded mes- and pros-encephala 

 having their bony ring or arch completed by a centrum below and 

 a spine above. One neurapophysis (alisphenoid) transmits the 

 trigeminal nerve, the other (orbitosphenoid) the optic nerve : the 

 fourth or anterior neural arch ( f os en ceinture ' and ( ethmoide ' 

 of Cuvier) encompasses the foremost segment of the brain 

 as the exoccipitals do the hindmost ; and they give passage 

 to the olfactory nerves. Ossification of this ring of bone begins 

 in its lateral halves : the essential relations and functions being 

 those which characterise the bones which in bony fishes will be 

 described as ( prefrontals.' Beneath, and supporting them, is a 

 pair of bones which may be regarded as a mesially divided 

 ' centrum ' (vomer) : and above is a pair of bones which may be 



1 cxxxix. ' Ce crane ne s'accorde pas plus avec la theorie des trois, des quatre, ou 

 des sept vertebres, meme avec celle d'une vertebre, qu'avec celle de 1'identite de 

 nombre des os,' vol. v. pt. ii. p. 391. 



