EFFERENT PATHS AND REFLEX ARCS 



329 



accessory, and cervical spinal nerves send their axons to the muscles that move 

 the head and eyes. 



This arc is concerned with the reflex regulation of the combined movements 

 of the head and eyes in response to the vestibular stimulation which results from 

 every movement and change of posture of the head. Strong stimulation of the 

 semicircular canals, vestibular nerve, or Deiters' nucleus causes an oscillatory 

 side to side movement of the eyes, known as nystagmus, a reflex response of an 

 abnormal character mediated through this arc (Wilson and Pike, 1915). 



M. rectus medialis 



Oculomotor nerve 



Vestibular nerve ' 

 Lateral vestibular nucleus - 



Vestibules pinal tract ' 



Median longitudinal --' 

 fasciculus 



M. sternocleidomas- _ 

 toideus 



M. rectus lateralis 



Nuc. of oculomotor nerve 

 Abducens nerve 



Nuc. of abducens nerve 



Median longitudinal 

 fasciculus 



Spinal root of accessory 

 nerve 



t N. ceroicalis II 



Fig. 244. Vestibular reflex arcs. (Modified after Edinger.) 



A vestibules pinal reflex -arc is established between the vestibular sense organs 

 and the skeletal musculature and consists of the following parts : the vestibular 

 nerve; the vestibulospinal tract, which has its origin in the lateral vestibular 

 nucleus, and descends in the ventral funiculus of the same side of the spinal 

 cord; and the primary motor neurons of the spinal cord (Fig. 244). 



The afferent impulses reaching the medulla oblongata by way of the vagus 

 give rise to a great variety of reflexes. While these are for the most part purely 

 visceral, a few are executed by the somatic musculature and should receive 

 attention at this point. 



