310 THE NERVOUS SYSTEM 



into the reticular formation of the pons, cross the median raphe to join the trape- 

 zoid body and enter the lateral lemniscus. While this tract is for the most part 

 a crossed one, some fibers probably enter the lateral lemniscus from the cochlear 

 nuclei of the same side. This accounts for the fact that it is very rare to have 

 total deafness in either ear resulting from damage to the auditory pathway 

 within the brain. The fibers of this fillet give off collaterals to the nucleus of 

 the lateral lemniscus, from which some additional fibers may be contributed to 

 the tract, which finally terminates in the medial geniculate body and the inferior 

 colliculus of the corpora quadrigemina. The latter, however, serves only as a 

 reflex center, while the medial geniculate body is the way station on the 

 auditory path to the cerebral cortex. 



Neuron III. Through synapses in the medial geniculate body the auditory 

 impulses are transferred to neurons of the third order, whose cell bodies are 

 located in this nucleus and whose fibers run through the auditory radiation 

 and the retrolenticular part of the internal capsule to the auditory receptive 

 center in the cerebral cortex. It will be remembered that this center is situated 

 in the anterior transverse temporal gyms, located upon the dorsal surface of 

 the temporal lobe within the lateral cerebral fissure, and in the small portion 

 of the superior temporal convolution with which that gyrus is directly continuous. 



The Neural Mechanism for Sight. The nervous impulses responsible for 

 vision travel over a conduction system composed of at least four units. Since 

 this mechanism has already been considered as a whole on pages 225-228 

 it is only necessary for us to enumerate here the separate units of which it is 

 composed (Figs. 160, 162). 



Neuron I. Visual cells of the retina including the rods and cones, which are 

 differentiated as receptors for photic stimuli. 



Neuron II. Bipolar cells of the retina, forming synapses with the visual 

 cells, on the one hand, and the ganglion cells on the other. 



Neuron III. Ganglion cells of the retina, whose axons enter the optic nerve, 

 undergo a partial decussation in the optic chiasma, and end in the lateral genic- 

 ulate body, pulvinar of the thalamus, and superior colliculus of the corpora 

 quadrigemina. 



Neuron IV. From cells in the lateral geniculate body and the pulvinar of 

 the thalamus axons run by way of the optic radiation through the retrolenticular 

 part of the internal capsule to the visual receptive center in the cerebral hemi- 

 sphere. This is located in the cortex on both sides of the calcarine fissure and 

 occupies portions of the cuneus and the lingual gyrus. 



