THE STRUCTURE OF THE BRAIN STEM 



closely connected with the co-ordination of visual and auditory i m 

 with the eye movements. 



Betters' nucleus, which occurs in the same region, although described a 

 one of the nuclei of the eighth nerve, might equally well be included in this 

 class owing to its manifold connections with both afferent and efferent 

 mechanisms. 



In close connection with Deiters' nucleus are a number of grey masses 

 in the cerebellum, the roof nuclei in the roof of the fourth ventricle. 



In the mid-brain we must mention the superficial grey matter covering 

 the corpora quadrigemina. 



On the ventral side of the Sylvian iter are the various masses of grey 

 matter in the crura, the red nucleus, a large mass in the tegmentum just below 

 the oculo-motor nucleus, and the substantia nigra, which divides each crus 

 into two parts, the dorsal tegmentum and the ventral pes or crusta. 



Finally at the fore part of the cerebral axis we come to the great ganglionic 

 mass already described, the optic thalamus and the geniculate bodies. The 

 geniculate bodies may be regarded as outgrowths of the optic thalamus 

 which have developed in connection with the terminations of the auditory and 

 the optic nerve fibres. The optic thalamus is connected by fibres with all 

 parts of the cortex and represents the termination of the whole tegmental 

 system, so that in many ways it may be regarded as a sort of foreman of the 

 central nervous system, controlling the activities of the lower level centres 

 and bringing all parts of this system in relation with the supreme cerebral 

 cortex. 



THE CHIEF LONG PATHS IN THE BRAIN STEM 



In dealing with the spinal cord we were able to treat it as one organ, 

 very largely on account of the uniformity of the afferent and efferent 

 mechanisms connected with its various segments. Every afferent impulse 

 arriving at the cord has many possible paths open to it, on account of the 

 branching of the nerve fibres as they enter the cord and the connection of 

 these branches with different neurons of varying destination. The exact 

 path taken by any given impulse under any given set of circumstances 

 is determined by the varying resistance at the synapses which intervene 

 between the terminations of the afferent fibres conveying the impulse 

 and the next relay of neurons. These resistances in their turn are altered 

 by the processes of facilitation and inhibition, which may be due to 

 contemporaneous or previous events. A conspicuous example of these 

 conditions is afforded by the phenomena of simultaneous and successive 

 spinal induction. 



The uniformity of afferent and efferent mechanisms disappears when wr 

 include the brain stem with the spinal cord. The main efferent channel 

 of impulses is still through the spinal cord, since here are found the efferent 

 mechanisms for all the skeletal muscles of the trunk and limbs, the chief 

 servants of the central nervous system in the daily events of life, 

 efferent channels are added, which acquire special importance with the 

 growth of the upper brain or cerebral hemispheres. These mechanisms 



