THE SEGMENTAL VALUE OP THE CRANIAL NERVES. 165 



is independent evidence in favour of the head retaining a more primi- 

 tive condition than the body. Thus the skull, though subjected to 

 very extensive secondary modifications, is really in a more primitive 

 state than the vertebral column, for the skull represents the per- 

 manent retention of a condition, that of a continuous unsegmented 

 cartilaginous tube, which is only transitory in the case of the vertebral 

 column except in the lowest vertebrates ; the division of the cartilagi- 

 nous tube into segments or vertebrae never occurring, and in all pro- 

 bability never having occurred in the skull, though so constantly 

 present in the vertebral column. The fact that it is in the lowest 

 vertebrates alone that this unsegmented condition is retained in the 

 trunk as well as in the head is a strong argument in favour of the 

 view that the head is really in a more primitive condition than the 

 trunk as regards skeletal elements. 1 



On the second hypothesis, the mixed — motor and sensory — nature 

 of the seventh nerve is explained as due, like the persistence of the 

 primary root and the independence of the sixth nerve, to retention of 

 the primitive condition, and the extreme variability presented by the 

 relative importance of the sensory and motor functions of the seventh 

 nerve in different vertebrates may help to render intelligible how the 

 posterior spinal roots, which were originally of mixed function, have 

 become converted into purely sensory roots. 



If the hypothesis advanced above should prove correct, it would be 

 only reasonable to expect that the posterior roots of the spinal nerves 

 - should in some exceptional cases be found to retain in part their primi- 

 tive mixed character, and to co-exist as mixed posterior roots with 

 exclusively motor anterior roots. I am not aware of any such cases, 

 or of the existence of any residual physiological phenomena that would 

 support such a view, but would suggest that a direct investigation of 

 the functions of the spinal roots in the lampreys, where the two roots 

 are stated to remain distinct from one another throughout life, might 

 conceivably lead to interesting results. 



The application of the hypothesis to the remaining cranial nerves 

 is sufficiently obvious from the accounts given of these. The main 

 point of difficulty concerns the determination of the presence or ab- 



1 The following quotation from Balfour, which I only became acquainted with after the 

 above passage was written, strongly confirms this view :— " This development (of the skull) 

 probably indicates that the basilar plate contains in itself the same elements as those from 

 which the neural arches and the centra of the vertebral column are formed, but that it never 

 passes beyond the unsegmented stage at first characteristic of the vertebral column,"— Com- 

 parative Embryology, vol. ii., p. 467, 



