NERVE 119 



2, Connections with the Central Nervous System. 



The fibres of the vestibular root take origin in dendrites 

 between the cells of the macula3 in the ampullae of the 

 semicircular canals and of the saccule, and have their nerve 

 cells upon then' course (fig. 56). 



They enter the medulla ventrally to the auditory nerve, 

 along with which they are described as the eighth cranial 

 nerve. 



They have wide and important central connections which 

 may be divided into four arcs : — 



(1) Labyrintho-Spinal Arc. — As the fibres enter the medulla 

 they divide and run upwards and downwards. Those pass- 

 ing down form synapses witli the higher spinal arcs which 

 take part in spinal reflex actions. From the upward and 

 down-cfoinsr branches, collaterals enter the nucleus of Deiters 

 lying in the side of the pons Varolii and there form synapses. 

 From these fresh neurons send fibres down the cord on the 

 same side and on the opposite side to act upon the spinal 

 arcs. 



(2) Labsnrintho-Oculo-Motor Arc. — This is essentially a part 

 of the spinal arc, but it is so important that it may be dealt 

 with separately. The ingoing fibres form synapses in Deiters' 

 nucleus, from Avhich fibres pass to act upon the oculo-motor 

 nuclei, and thus to influence the movements of the muscles 

 of the eyes (p. 161). 



(3) Labyrintho-Cerebellar Arc. — Some of the upgoing fibres 

 pass to the deep nuclei of the cerebellum, from which fibres 

 pass down the spinal cord to act upon the spinal reflex 

 arcs. 



(4) Labyrintho- Cerebral Arc — Other upgoing fibres pass to 

 the cerebrum forming synapses in the optic thalamus, from 

 which fibres pass on to the cortex. From this, fibres extend 

 down the cord to act upon the spinal reflex arcs and upon 

 the oculo-motor mechanism. 



