THE SENSORY CORTICAL AREAS 655 



the auditory nerves. The fibers of this nerve leaving the ganglion cells of the 

 cochlear nucleus are carried by the lateral fillet to the posterior quadrigeminal 

 body (Flechsig and Bechterew). This is abundantly connected with the in- 

 ternal corpus geniculatum, which in turn is connected exclusively with the 

 cortex of the temporal lobe (v. Monakow). According to Flechsig, the two 

 transverse convolutions of this lobe represent the substations of the auditory 

 nerve. 



These convolutions lie deep in the fissure of Sylvius, where they push in 

 between the posterior border of the island of Reil and the outer free surface 

 of the first temporal convolution. The fact that in all cases of total deafness 

 as the result of bilateral destruction of the human cortex thus far known, 

 this region of the two transverse convolutions was affected, speaks strongly 

 for its importance as an auditory cortical area. Cases of deafness or of dull 

 hearing on one side following injury to this region, or to its coronal radiation 

 or to its fibers in the internal capsule, furnish evidence to the same effect. 



D. THE VISUAL AREA 



Experimental as well as clinical and anatomical evidence indicates that the 

 cortical area for the optic nerve is to be sought chiefly in the occipital lobe. 

 Statements differ a great deal as to the exact boundaries of this area, owing 

 in part at least to the fact that in some animals the localization is sharper 

 than in others. 



In the dog, according to H. Munk, the two retina? are projected upon the 

 occipital lobes in the following manner. The extreme lateral part of each 

 retina is represented by the extreme lateral surface of the occipital lobe on 

 the same side. But by far the greater part of each retina is represented by the 

 remaining greater part of the occipital lobe on the opposite side, the inner 

 edge of the retina corresponding to the median edge of the occipital lobe, 

 the upper edge of the retina to the anterior edge of the lobe, and the lower 

 edge to the posterior edge. 



As opposed to this Goltz, among others, has observed after bilateral extir- 

 pation of the occipital lobe, that while a great reduction of the visual power 

 and a very considerable loss of intelligence may result, the animal still cannot 

 be called totally blind. For, although he may not respond to a threat with the 

 hand or with a light, he still is able to avoid obstacles fairly well without 

 being guided in any way by the sense of touch. These observations show that 

 the animal in this very low mental condition either receives visual sensations 

 through the remaining parts of the cortex, or that the movements can be 

 regulated by retinal impulses with the help of the subcortical centers. 



Several authors, however, have observed that in the dog a temporary reduc- 

 tion of the visual power on the corresponding halves of the two eyes (homolateral 

 hemiamblyopia) may result from the removal of other cortical regions (e. g*., the 

 motor zone). One would be inclined to conclude from this that while most of 

 the fibers from the optic tracts reach the occipital lobe, some of them have ter- 

 mini in other parts of the cortex. But the following observations by Hitzig, 

 which have recently been confirmed in their entirety by Exner and Imamura, 

 prove that the relationship is still more complex. If a part of the occipital cortex 



