212 PHYSIOLOGY <pF CENTRAL NERVOUS SYSTEM. 



eyes downward would imply stimulation of the upper half of the 

 retina, since objects in the lower part of the visual field form their 

 image on the upper half of the retina. This fact, that stimulation 

 of the occipital cortex causes definite movements of the eyeballs, 

 seems to imply that there are efferent fibers in the occipitothal- 

 amic radiation running from the occipital cortex to the midbrain, 

 where they make connections with the motor nuclei of the third, 

 fourth, and sixth cranial nerves. 



The Function of the Lower Visual Centers. The first ending 

 of the optic fibers lies in the lateral geniculate and to a lesser 

 extent in the thalamus and superior colliculus. It is conceiv- 

 able, of course, that some degree of visual sensation may be 

 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 



