THE SENSORY CORTICAL AREAS 657 



In the monkey the observations of H. Munk, Brown and Schafer and others 

 agree in showing that extirpation of one whole occipital lobe results in loss 

 of vision on the corresponding halves of the two retinae (homolateral hemi- 

 anopia, Fig. 295), and bilateral extirpation in total blindness. According to 

 H. Munk, there should be a projection of the retina upon the occipital lobes 

 of the monkey like that described above for the dog. But Brown and Schafer 

 have not obtained any positive results in this direction by partial removal of 

 the occipital lobes. 



The clinical evidence is perfectly clear that in man the cortical area for 

 the optic nerve is situated in the occipital lobes. A sufficiently extensive lesion 

 of the occipital cortex is followed just as in the monkey by homolateral hemi- 

 anopia on both sides. As a rule, this is not complete, for the line of separation 

 leaves the central part of the visual field intact. In certain individual cases 

 of bilateral hemianopia accompanying lesions of both occipital lobes, the 

 portion corresponding to the yellow spot may remain entirely free. 



Opinions differ considerably as to the exact location of the visual area 

 in the occipital lobe. According to Nothnagel, it is coterminous with the 

 cuneus and the first occipital convolution ; according to Vialet, with the whole 

 median surface of the occipital lobe. Still others extend it further, to the 

 first and third occipital convolutions, or even to the angular gyrus, which 

 latter, according to Ferrier, is the region for distinct vision. As opposed to 

 these, and on the strength of some very convincing cases, Henschen in par- 

 ticular advocates the view that only the cortex along the calcarine fissure is 

 to be regarded as the area of vision. 



Flechsig, on the basis of his embryological studies, takes very much the 

 same view. Most of the optic fibers end in the wall of the calcarine fissure, 

 and those regions of the visual area situated outside of this limited tract have 

 but a limited share in the true visual process. 



The visual conducting paths, according to most investigators, take the 

 following course to the occipital cortex. The optic fibers springing from the 

 ganglion cell layer of the retina pass to the chiasm; those corresponding to 

 the outer lateral parts of the retina remain uncrossed; the remainder cross. 

 With their end arborizations some come into relation with the ganglion cells 

 of the anterior quadrigeminal body; many more, and among them the fibers 

 from the macula, with the cells of the external corpus ; and a smaller number 

 with cells in the pulvinar. New pathways spring from these various cells 

 and make their way to the occipital lobes. 



In the opinion of v. Monakow, the reason the macula region so often 

 remains intact in cerebral lesions is that it is probably represented throughout 

 by a rather extensive cortical zone ; the macula fibers then would be connected 

 with practically all parts of the corpus geniculatum ext. ; consequently, if the 

 lesion left any fibers to the cortex intact, impulses from the macula could 

 still be transmitted. 



According to Flechsig, efferent fibers pass out from the occipital lobes, and 

 convey impulses from the cortex to the optic thalamus and the anterior corpus 

 quadrigenum by way of which impulses can be conveyed from the optic lobes 

 to various muscles and other peripheral organs. 



