THE STRUCTURE OF THE BRAIN STEM 381 



systems of nuclei and conducting paths), which have arisen in consequence 

 of the great complexity of reaction required of the nerve mechanisms in 

 connection with the organs of special sense. We must confine ourselves 

 here to little more than the enumeration of the chief masses, though we 

 shall have occasion to refer to some in more detail when dealing with the 

 co-ordinating mechanisms of the cerebral axis. From below upwards we 

 may enumerate the following grey masses : 



In the medulla is the large olivary body, with the accessory olive lying on 

 its inner side. Each olive sends fibres across the middle line to the opposite 

 cerebellar hemisphere, and must be regarded as connected with this body 

 in its functions, since atrophy or removal of one side of the cerebellum is 

 followed by atrophy of the opposite olive. 



In the pons we find a similar but smaller body, the superior olive, in the 

 neighbourhood of the nucleus of the seventh nerve. The superior olive is 

 closely connected with the co-ordination of visual and auditory impressions 

 with the eye movements. 



Deiters' nucleus, which occurs in the same region, although described as 

 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 



