SENSE AREAS AND ASSOCIATION AREAS. 197 



no cases are reported of cortical lesions involving only or mainly 

 the vision in the macular region. He therefore argues that, while 

 the paths from the retina to the lower visual centers (external 

 geniculate) may be isolated, the further connections with the 

 cortex must be widespread. The cortical center for distinct vision 

 according to this view is not limited to a narrow area, but must 

 involve a large region in the occipital cortex. It is difficult to 

 reconcile this view with the ideas of isolated conduction and specific 

 function of each part of the cortex, and it is very evident that the 

 projection of the retinas upon the cortex is a question that must 

 be left for further observation and experiment. Some light was 

 supposed to be thrown upon the subject from the results of stimula- 

 tion of the occipital cortex. Stimulation of this kind causes move- 

 ments of the eyes, and the movements vary with the place stimu- 

 lated.* Stimulation of the upper border of the lobe causes move- 

 ments of the eyes downward, stimulation of the lower border move- 

 ments upward and of intermediate regions movements to the side. 

 Assuming that the direction of the movement is associated with 

 movements toward that part of the visual field from which a 

 normal visual stimulus would come, it is evident that movements 

 of the 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. Following this 

 suggestion, the projection of the retinas on the occipital lobes, or 

 the cortical representation of the retinas on the occipital cortex 

 might be represented by a definite schema. Such a definite rela- 

 tionship, however, as stated above, is not borne out by clinical facts. 

 The fact that stimulation of the occipital cortex causes definite 

 movements of the eyeballs seems, however, to be demonstrated and 

 it implies that there are efferent fibers in the optic radiation running 

 from the occipital cortex to the midbrain, where they make con- 

 nections 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 external geniculate and to a 

 lesser extent in the thalamus and superior colliculus. It is con- 

 ceivable, 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 me- 

 chanical 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 posterior longitudinal 

 * Schafer, "Brain," 11, 1, 1889, and 13, 165, 1890. 



