198 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



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 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 proc- 

 esses 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 con- 

 nections 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 development 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- 

 gators vary considerably. The eighth cranial nerve springs from the 

 bulb by two roots: the external and the internal. The former has 



