210 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



mediated through these centers. Goltz observed that in dogs 

 with the cerebrum removed the animals showed a constriction 

 of the pupils when a bright light was thrown upon the eyes or 

 even closed the eyes. It is the general belief that reactions of 

 this kind are mechanical or unconditioned reflexes accompanied by 

 no higher psychical reaction than in the case of spinal reflexes. 

 The existence in the midbrain of the motor nuclei of the third nerve* 

 and of the medial longitudinal fasciculus through which con- 

 nections 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 mem- 

 ories, 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, how- 

 ever, the development of a cerebral cortex is followed by the 

 evolution of the occipitothalamic radiation, and as the connec- 

 tions 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 por- 

 tions of the brain lying most anteriorly, and doubtless the degree 

 of consciousness is greatly intensified in the higher animals in cor- 

 respondence 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 cortical terminus 



