THE SENSORIAL CORTICAL CENTERS. 787 



the largest number of fibers from the opposite cortical center, and fibers only 

 for the outermost lateral marginal portion from the center of the same side. If 

 the surface of one retina be conceived as projected upon the centers, the outer- 

 most margin of the former will be connected with the center of the same side, 

 the inner margin of the retina with the inner portion of the opposite center, the 

 upper marginal portion with the anterior portion, and the inferior marginal 

 portion of the retina with the posterior portion of the opposite center. The 

 (shaded) middle of the center corresponds to the point of direct vision of the 

 retina of the opposite side. 



Irritation of the visual center causes in the dog movements of both eyes 

 toward the opposite side, at times with movements of the head of like character 

 and contraction of the pupils. If an eye is excised from a new-born dog the contra- 

 lateral psycho-optic center will after an interval of months be found to be less 

 well developed. After extirpation of the visual sphere in young animals the 

 external geniculate body, the pulvinar (Fig. 263), the anterior quadrigeminate 

 body (of the same side and in part also of the opposite side), undergo atrophy, 

 together with degeneration of the sensory sphere for the eye (Fig. 259), and at 

 a later period also the optic tract and nerve undergo atrophy. A similar condi- 

 tion has been observed after degeneration of the visual sphere in man. 



The situation of the visual center has been outlined in a different manner by 

 different investigators. According to Ferrier it is located in the dog in the region 

 of the occipital portion of the third primitive convolution indicated by e e e (Fig. 

 258) , and according to more recent statements in the occipital lobe and the angular 

 gyrus. 



According to Luciani, the visual field includes, in addition to the occipital, 

 also the parietal lobe (in the dog and the ape) . He also dissents from the precise 

 projection of the retinas upon the cerebral cortex. He believes that both optic 

 nerves are connected with all portions of the occipitoparietal region. Moreover, 

 he is of the opinion that the visual images are only transformed in the cortex 

 psychically, but that they arise in the quadrigeminate body, as he admits, even 

 after most extensive destruction of the occipitoparietal region on both sides, 

 the development only of mind-blindness, but not permanent actual blindness. 

 Even previously Christiani had maintained that rabbits deprived of their cere- 

 brum still avoided obstructions in running, because they were able to appreciate 

 them with their eyes. In such animals, therefore, optic impressions must be 

 advantageously utilized, and in such a manner that the optical impressions so 

 affect the chief reflex and the coordination-center in the optic thalamus that the 

 animals make appropriate reflex movements. Conscious vision is thus lost, while 

 the coordinated reflex activities excited from the visual apparatus are still pre- 

 served. 



In apes the center is situated at the apex of the occipital lobe. Destruction 

 of the center on one side causes blindness for the halves of both retinas upon the 

 side of the injury. In birds the visual sphere is situated in the portion of the 

 cerebral cortex extending from the peduncle upward and forward and covering 

 the ventricle. The retina of the opposite eye is supplied from one hemisphere, 

 with the exception of its most posterior portion, which is supplied from the hemi- 

 sphere of the same side. In the frog the visual center is situated in the optic 

 lobe; frogs and fish thus see without a cerebrum. 



The psycho-auditory center or the auditory sphere is situated in the dog in 

 the region of the second primitive convolution indicated by the letters f f f (Fig. 

 258, II) .according to Munk in that portion of the temporal lobe marked " Hearing " 

 (Fig. 259). Destruction of the entire region causes deafness in the contralateral 

 ear; destruction of the middle shaded portion alone causes mind-deafness or audi- 

 tory amnesia, that is the animal has lost the memory-images of auditory impres- 

 sions. Irritation of the center is followed by a reaction that corresponds to the 

 abrupt start induced by a sudden and unexpected loud noise. Irritation of the 

 center on one side causes movement of the ear on the opposite side. Under these 

 circumstances also, the disturbances attending injury of the middle portion 

 on one side disappear in the course of a few weeks (as in the case of the psycho- 

 optic center) , so that the animal must again learn to hear. Destruction of the 

 middle portion on both sides causes mind-deafness on both sides. Dogs thus 

 injured no longer prick their ears in response to auditory impressions and they 

 gradually lose the faculty of barking. The anterior portions of the auditory 

 sphere appear to subserve the perception of high notes and the posterior portions 

 the perception of deeper notes. Munk observed after destruction of one ear in 



