390 



On the Structure, of the Cranial Nerves. [Feb. 23, 



freshness and freedom from damage which is requisite to ensure a 

 good osmic preparation, has prevented me up to the present from 

 making sure of this latter point. 



When the facial leaves the stylo-mastoid foramen it is free from 

 these large fibres. I venture to suggest that the structure of these 

 two muscles, their origins, their shape, colour, and appearance, 

 combined with the size of their motor fibres, all lead to the conclusion 

 that they belong to the same group as the somatic eye muscles, and 

 represent van Wijhe's missing 4th and 5th myotomes. Further, we 

 see this, that if the splanchnic voluntary muscles derived from the 

 lateral plates of mesoblast are differentiated from the somatic voluntary 

 muscles derived from the myotomes by the size of their motor nerve 

 fibres, we ought to find the same relation in the trunk as in the head ; 

 this seems to me to be the case, the nerve fibres of the phrenic in the 

 rabbit separate out from those of the 4th and 5th cervical nerves as a 

 group of fibres of smaller size than the surrounding motor nerves of 

 those segments, and also the fibres of the nerve supplying the 

 w. transversus abdominis in the dog are distinctly smaller than those of 

 the nerve supplying the m. obliq. sup. I conclude then that the 

 primitive segmentation of the head is shown by the somatic muscles 

 of the Illrd, IYth, and Vlth nerves, m. stapedius, m. levator veli 

 palati, and the muscles of the Xllth nerve, a segmentation which is 

 in agreement with the segmentation of the trunk. In addition to 

 this a secondary segmentation has taken place in the formation of the 

 gills ; the muscles belonging to this segmentation show a segmental 

 arrangement in connexion with the gills but not in the case of the 

 trunk, for as far as I know the m. transversus and the diaphragm show 

 no sign of segmentation. 



In this preliminary communication I cannot discuss all the 

 problems which are opened out by the new light which examination 

 of the structure of the cranial nerves has shed upon their distribution 

 and past history. The explanation of the degenerated posterior 

 roots of certain of the cranial nerves is certainly to be found in the 

 history of the vertebrate animal, and I hope in the course of the 

 summer to publish the full paper of which this is a preliminary 

 account, and in that paper to make some attempt to account for this 

 remarkable phylogenetic degeneration. In conclusion, I may remark 

 that Marshall has previously noticed in the chick a group of ganglion 

 cells at the origin of the Illrd nerve out of the brain. Also it has 

 been pointed out to me that my discovery of degenerated nerve cells 

 and nerve tissue in such cranial nerves as the Illrd is not, as I 

 thought, entirely new. The structures in question have been 

 observed by Thomsen and other pathologists. The explanation 

 which I have given is, however, entirely new. 



