Johnston, Moj-pJiology of the Head. 231 



homologous ; he does not think of the possibihty of two separ- 

 ate kinds of dorsal roots being found in the same segment under 

 some conditions. Therefore if nerves of spinal type are found 

 in the vagus region they must have reached their present posi- 

 tion by shifting through the long branchial region. As the 

 above discussion has shown, the dorsal and ventral "hypoglossal" 

 roots need not be considered as spinalajiige nerves. They 

 probably are not equivalent to spinal nerves at all but are only 

 the general cutaneous and somatic motor components of nerves 

 of the vagus region, the visceral^ensory and motor components 

 of which have been collected into the single large vagus root. 

 The presence of these nerves in the vagus region, then, does not 

 require the hypothesis that they have shifted forward from the 

 postbranchial region but is directly opposed to such a hypothe- 

 sis. So far as these nerves are concerned a shifting forward of 

 trunk elements into the head simply has not occured. The 

 nerves are just where we should expect to find them if no shift- 

 ing or other modification has taken place. This fact is of course 

 not inconsistent with the observed shifting forward of trunk 

 myotomes and the fusion of vertebral elements with the skull. 

 As stated elsewhere, the disappearance of the spino-occipital 

 nerves is due to the reduction of their innervation territory con- 

 current with the shifting forward of trunk elements. The struc- 

 ture of the nervous system is opposed, however, to the view 

 which supposes that the vertebrate head once (palingenetic) in- 

 cluded all the structures innervated by the vagus and the seg- 

 ments in which they were situated, and that all structures now 

 found in the vagus region and not innervated by the vagus have 

 moved forward from the trunk. Owing to the formation of a 

 rigid cranium to protect the brain and sense organs and the 

 secondary extension of this cranium farther caudad by the 

 fusion of more or fewer vertebral elements to the cranium, there 

 has been a reduction of unnecessary myotomes and a shifting 

 forward of trunk myotomes to gain attachment to the cranium. 

 This may be ground for recognizing a coenogenetic cranium ; but 

 the cranium is not the head. The shifting of any organ is 

 relative to all the others. The expansion of the branchial ap- 



