84 Dcvclopinciit of Occipital Xcrves in Human I'^nibryos 



sensory roots with root ganglia derived from tlie same ganglion crest. 

 As the development progresses the cranial end of this complex becomes 

 predominantly sensory and the candal end predominantly motor, and 

 also more spread ont which gives rise to a difference in the appearance 

 of the two portions in the adnlt, and has resulted in their being con- 

 sidered as two independent strnctnrcs. 



That such a relation between the tenth and eleventh cranial nerves 

 exists is not a new idea, but was long ago suggested by the work of His, 

 88, on the human embryo, though this investigator did not work with 

 sufficiently young stages to make the evidence conclusive. The theory 

 has since then been supported by the work of Furhringer, 97, and 

 Lubosch, 99, who believe that phylogenetically the tenth and eleventh 

 nerves cannot be separated. Cliiarugi, 90, however, from the compara- 

 tive embryology of these structures, concludes that the eleventh is not a 

 part of the tenth, but is a nerve for itself which results from the differen- 

 tiation of the nucleus of origin of the ventral roots into median and lat- 

 eral divisions ; the latter rootlets losing their segmental distribution take 

 a new course and depart obliquely through the cranium as an independent 

 nerve. Another view regarding these nerves is offered by Minot, 92. 

 He suggests that a modification may have occurred in the motor fibres 

 of the dorsal roots of the hypoglossus, by which these motor fibres, 

 following the abortion of the ganglia, no longer join the ventral roots 

 of the twelfth, but turn forward to join the vagus thereby forming the 

 tnmk of the accessory nerve. Since the work of His, 88, and Mall, 91, 

 on the human embryo, further details in the development of the tenth and 

 eleventh nerves in other mammals have been supplied in the well known 

 papers of Froriep, 83, 85, and 01, and by the work of Robinson, 92, and 

 Letuis, 03. The latter two give us a more accurate description of the 

 so-called ganglionic commissure, than had before existed; although they 

 failed to recognize the full significance of these ganglia and their relation 

 to the precervical ganglia of Froriep. 



The comparative morphology of the occipital nerves, particularly with 

 regard to their bearing on the segmental origin of the head, has been the 

 subject of much speculation, ever since Oegenbaur, 72, published his 

 work on the selachian head. The charge may perhaps be justly made 

 that more space in the literature is given to theories and discussions 

 concerning these structures than to actual observations on their com- 

 parative and embryological anatomy. This subject will be briefly treated 

 under the heading comparative morphology. It will be emphasized there 

 that the ganglia of the trunks of the ninth and tenth (gang, petrosum 

 and gang, nodosum) are branch io-meric and largely independent of the 



