200 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



in the case of spinal reflexes. The existence in the midbrain of the 

 motor nuclei of the third nerve, and of the posterior longitudinal 

 bundle through which connections are established with the motor 

 nuclei of other cranial nerves, furnishes us with a possible reflex 

 arc through which the visual impulses brought into the lower optic 

 centers, especially the superior colliculus, may cause co-ordinated 

 movements of the eyes or of the head. Usually it is assumed that 

 conscious visual sensations, and especially visual associations and 

 memories, are aroused only after the impulses reach the occipital 

 cortex. In the fishes the midbrain forms the final ending of the 

 optic fibers, and in these animals, therefore, whatever psychical 

 activity accompanies the visual processes must be mediated through 

 this portion of the brain. In the higher animals, however, the 

 development of a cerebral cortex is followed by the evolution of the 

 optic radiation, and as the connections of the occipital cortex 

 increase in importance, those of the midbrain (with the optic fibers) 

 dwindle correspondingly. Here, as in other cases, the psychical 

 activity is concentrated in the portions of the brain lying most 

 anteriorly, and doubtless the degree of consciousness is greatly 

 intensified in the higher animals in correspondence with the devel- 

 opment of the cerebral cortex whose striking characteristic is its 

 capacity to evoke a psychical reaction. 



The Auditory Center. The location of the auditory area has 

 been investigated along lines similar to those used for the visual 

 center. The experimental physiological work has yielded varying 

 results in the hands of different observers. Munk and Ferrier 

 placed the cortical center for hearing in the temporal lobe, and 

 in spite of negative results by Schafer and others this localization 

 has been shown to be substantially correct. Entire ablation of 

 both temporal lobes is followed by complete deafness. Ablation 

 on one side, however, is followed only by impairment of hearing, 

 and in the light of the results from histology and from the clinical 

 side it seems probable that the connections of the auditory cortex 

 with the ear follow the general schema of the optical system rather 

 than that of the body senses. That is, it is probable that the 

 auditory fibers from each ear end partly on the same side and 

 partly or mainly on the opposite side of the cerebrum. The exact 

 portion of the temporal lobe that serves as the immediate organ 

 of auditory sensations cannot be determined with certainty, but 

 it seems certain that it lies mainly. in the superior temporal gyrus, 

 and the transverse gyri extending 'from this into the fissure of 

 Sylvius. 



The Histological Evidence. On the histological side the paths of 

 the auditory fibers have been followed with a large measure of suc- 

 cess, although in many details the opinions of the different investi- 



