THE CRANIAL NERVES AND THEIR NUCLEI 1 70 



This group of small cells gives origin to the general visceral efferent fibers of the 

 oculomotor nerve which run to the ciliary ganglion for the innervation of the 

 intrinsic muscle of the eye. 



Neurobiotaxis. The position of the motor nuclei of the brain stem varies greatly in 

 different orders of vertebrates, and is determined by the source of the principal afferent 

 impulses which reach them. The perikarya of the neurons migrate under the influence of an 

 attraction, which has been called neurobiotaxis, in the direction of the chief fiber tracts 

 from which they receive impulses (Ariens Kappers, 1914, 1917; Black, 1917). "When from 

 different places stimuli proceed to a cell, its chief dendrite grows out and its cell body shifts in 

 the direction whence the majority of the stimuli proceed," while the axon grows in the op- 

 posite direction (Fig. 127). The nature of the attractive force is not altogether clear. Kap- 



Cell 



Axiscyh 



B 



Fig. 127. Diagram to illustrate the principle of neurobiotaxis. The axis-cylinder grows in 

 the direction of the nervous current, indicated by the arrow, while the dendritic outgrowth and 

 the final shifting of the cell body occur against the nervous current : A , Dendrites grown out to- 

 ward the center of stimulation; B, the cell body has shifted toward the center of stimulation; the 

 axis-cylinder is consequently elongated. (Kappers.) 



pers believes that it is a galvanotropic phenomenon, on the basis of the fact that the stimu- 

 lation center is electrically negative, i. e., a cathode with reference to the surrounding tissue. 



Numerous instances might be cited of the action of this taxis, but two will suffice. It 

 has already been noted that the eye-muscle nuclei receive most of their collaterals from the 

 optic and vestibular reflex tracts; and these appear to be the most important factors in the 

 determination of the positions occupied by those nuclei. The changes in position of the nuclei 

 in the vertebrate series appear to run parallel to the changes in these tracts. The reader 

 will now appreciate the significance of the close relation of these nuclei to the medial longi- 

 tudinal and tectospinal fasciculi which convey to them impulses from the vestibular and 

 optic centers. 



The position of the nucleus of the facial nerve and the curved course of its fibers within 

 the pons may be explained in the same way. In a 10 mm. human embryo the nucleus of the 

 facial nerve lies rostral to that of the abducens and the motor fibers pass directly lateralward 



