THE STRUCTURE OF THE BRAIN STEM 431 



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 con- 

 nected 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 vestibular 

 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 cms 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 termina- 

 tions 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 



