THE SPECIAL NERVOUS MECHANISMS 



115 



hand, and the cerebellum on the other. There is, however, no 

 evidence that it has any connections with the cortex. 



The organ of hearing, called the " cochlea," is essentially a 

 long tube bent on itself like a hairpin, and twisted round spirally 

 like the shell of a periwinkle. Within this tube is a fluid which 

 bathes a very peculiar and complicated structure called the 

 organ of Corti. The fibres of the true auditory or cochlear nerve 

 terminate in cells in the organ of Corti, and delicate auditory 

 hairs project out from these cells into the fluid. When the 

 latter is set in vibration, the auditory hairs are stimulated 

 in such a way that 

 afferent impulses are 

 transmitted along the 

 fibres of the nerve into 

 two nuclei in the medulla, 

 where they are received 

 by synapses. The axons 

 of the cells in these nuclei 

 are now gathered up into 

 certain internal tracts, 

 which pass up along the 

 lemnisci (see Fig. 28) into 

 the mid - brain. Other 

 fibres go to the cerebel- 

 lum. From the cells in the 

 mid-brain round which 

 the axons of the lemnisci 

 end other axons go up, 

 along a very obvious tract, 

 called the auditory radia- 

 tions, into the sensory 

 region of the cortex. 



The Organs of Vision. 



We have already said 

 something about the general structure of the eyes. The essential 

 elements are the cells of the retina, which, we have seen, are 

 arranged in layers. Light is received by either the rods and 

 cones, and these are prolonged into axons which form synapses 

 with other " bipolar" cells, the axons of which form other synapses 

 with third, " ganglionic " cells, the axons of which pass out of the 

 retina into the optic nerve. The latter then run up towards the 



Corpora. 

 CjjUadrijgemina. 



Thalamus 



Optic 



" radiations 



FIG. 32. THE CONNECTIONS OF THE 

 RETINAS WITH THE BRAIN. 



Only the tracts on one side are shown. 

 The section of the brain represented dia- 

 gratnmatically passes rather obliquely 

 through the eyes. 



