Motor System 49 



visceral motor cells of those segments of the spinal cord in 

 which it lies (PI. \l.). It extends from near the level of the 

 posterior end of the hypoglossal nucleus through the upper 

 cervical segments of the cord. 



There remains to be considered one other special visceral 

 efferent centre, namely the motor trigeminal nucleus (nucleus 

 motoriusV, nucleus masticatorius) , which controls the move- 

 ments of the jaw muscles. This nucleus consists of a rather 

 conspicuous group of large cells which do not lie very close 

 together. It is situated medially to the chief sensory V 

 nucleus and gives rise to the motor fibres of the \^ nerve 

 (PI. XI.). These form large fascicles which may be observed 

 passing out in an antero-ventral and somewhat lateral direc- 

 tion. 



The various changes in position which we have just seen 

 to have taken place in the development (both ontogenetic 

 and phylogenetic) of the motor nuclei are prominent ex- 

 . amples of a principle which seems to be active throughout 

 the nervous system, and which has been called by Kappers 

 neurohiotaxis. The most essential part of this principle may 

 be summed up by saying that two nerve cells which are 

 simultaneously or consecutively excited exert an attractive 

 influence on each other, as a result of which the cell body 

 tends to migrate during development towards any other 

 neuron from which it receives stimuli. 



