

CBEEBRAL NERVES 



129 



Thus in Petroinyzon Koltzoff describes the segmental ganglia as originating from a mesial 

 rudiment derived from the ganglion -crest, and from two peripheral ectodermic rudiments derived 

 from the lateral and epibranchial placodes. In the region of the head these join to form the 

 permanent ganglia ; in the region of the trunk the surface placodes do not join with the 

 central ganglion-rudiments, but remain separate and form the lateral-line organs. 



VII 

 VIII a.v. IX X XI XII 



III _ 



1 Co. 



M-- v v 



1 L. 



PIG. 173. RECONSTRUCTION OF THE NERVOUS SYSTEM OF A HUMAN EMBRYO OF 10'2 MM. (After His.) 



v, ventricle of heart; liv, liver; v.L, vitelline loop of intestine; /, tail; rh, rhinencephalon ; 

 c./t., cerebral hemisphere ; dien, diencephalon ; mes, mesencephalon ; a.v., auditory vesicle ; 

 /*'/.//., Froriep's ganglion ; ph, phrenic nerve. The Roman numerals indicate the nerves. The sixth 

 cerebral nerve is not labelled, but is seen passing forwards to the eye under the mandibular and 

 maxillary branches of the fifth nerve. 



In embryos of the third week the rhombic brain, as has already been indicated, shows a 

 subdivision by slight folds into a series of divisions which have been termed neuromeres on 

 the supposition that they represent a definite segmentation of the neural axis. In the pig 

 Bradley finds the same number as Thompson in the human embryo viz. seven and states 

 that the cerebellum lies opposite the first and the auditory vesicle opposite the third segment, while 

 the trigeminal ganglia are related to the second, the acoustico-facial to the fourth, the glosso- 

 pharyngeal to the sixth, and the vagus to the seventh. Broman describes in addition an eighth 

 VOL. I. K 



