130 PROFESSOR MARSHALL. 



anticipated by more than twenty years Gegenbaur's announcement 1 

 of that theory of the vertebrate skull which has since, w r ith some 

 slight modifications, been accepted almost universally. 



While the school of morphologists we first dealt with determined 

 the number of the segmental nerves by that of the skull-segments, 

 Stannius showed conclusively that there was no relation whatever 

 between the tw T o, but that there "was a very definite and remarkable 

 one between the segmental nerves and the visceral arches. Gegenbaur 

 went a step further, and, starting with the segmental nerves and 

 visceral arches, determined from them the number of head-segments, 

 thus completely reversing the order of proceeding of the older school. 



Gegenbaur is sometimes credited with being the first to establish 

 the relations of the cranial nerves to the visceral arches, a determination 

 which, as we have seen, had been already made by Stannius. The 

 often quoted table of the cranial nerves given by Gegenbaur, 2 contains, 

 in fact, nothing that had not been already pointed out by Stannius, 

 except an attempt to rank the labial cartilages as visceral arches, an 

 attempt which has not met with general acceptance. Gegenbaur's 

 real merit consisted in pointing out that the ideal number of skxill- 

 vertebrae, as determined by Oken and other "transcendental anato- 

 mists," was to he left out of consideration altogether ; that the evidence 

 offered by the cranial nerves and visceral arches was to be accepted in 

 full, and w r as to be taken as the basis for determining the number of 

 segments in the head ; and that the vagus nerve was, from the fact 

 of its supplying more than one visceral cleft, to be considered as 

 equivalent to more than one segmental nerve, and to be regarded as 

 formed by the fusion of a certain number of primitively distinct nerves, 



Thus it has come to pass that the cranial nerves, while formerly 

 considered of very subordinate importance, are now recognised as 

 affording a very valuable and reliable clue to the solution of that 

 favourite morphological problem — the segmentation of the vertebrate 

 head ; and Gegenbaur's paper, which was undoubtedly the chief means 

 by which the cranial nerves were rescued from their former dependent 

 position, must be viewed as marking a most important era in its 

 history. 



Attention being thus pointedly directed to the cranial nerves, their 

 comparative anatomy and embryology quickly engaged the attention 



1 Gegenbaur, " Ueber die Kopfnerven von Hexancbus," Jenaische Zeitsckrtft, 1871. 



2 Gegenbaur, loo, cit. p. 552< 



