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NERVOUS SYSTEM 



have also been interpreted as being dorsal nerves. Fiirbringer (1902) puts the 

 trochlear in an intermediate position, but regards it as most resembling a lateral 

 motor root. His (1904) regarded both it and the oculo-motor as nerves springing 

 from a region in which the distinction between the columns is less sharp than 

 elsewhere ; he was not therefore inclined to lay special stress on the point. The 

 oculo-motor he held to be certainly a mesial (ventral horn) nerve. 



The lateral column includes the nuclei of the motor roots of the vagus and 

 spinal accessory, glossopharyngeal, facial, and trigeminal. The fibres supply the 

 visceral musculature. 



The sensory roots are developed from ganglia which arise from a forward 

 continuation of the common ganglion-crest, but they show no regular segmental 



FIG. 172. SECTION FROM THE SAME EMBRYO AT THE EXIT OF THE FACIAL NERVE. (His.) 

 (Several sections have been combined to form this figure.) 



VL, fibres of sixth nerve taking origin from group of neuroblasts in basal lamina ; VII.G.g., ganglion 

 geniculi of the facial; VIII.G.i.c., intracranial ganglion of auditory; VIII.G.v., ganglion vestibuli ; 

 VIILG.c., ganglion cochleae. 



disposition. Moreover, unlike the spinal nerve-ganglia, they come into transient 

 relation to thickened patches of the surface epithelium (placodes) which are placed 

 above the gill-arches and have been interpreted in two ways. According to one 

 view (van Wijhe, Beard, Froriep, and others), the placodes represent branchial 

 sense-organs which have been lost in phylogeny, and they take no share in the 

 formation of the cranial nerves. They occur in Selachians in two series, a lateral 

 and an epibranchial. According to a second opinion (Kupffer, Goronowitsch, 

 Julia Platt, Koltzoff, and others), these placodes are not rudimentary sense-organs, 

 but thickenings from which cells are budded off to share in the formation of the 

 definitive nerve-ganglia. 



