600 PHYSIOLOGY CHAP. 



the monkey, the whole of the occipital lobe and the angular gyms 

 of one or of both sides be extirpated. Terrier arid Yeo (1880) 

 came to approximately the same conclusions in a further series of 

 researches on the monkey. 



On the other hand, Munk, in subsequent communications 

 (1880-81), developed his famous theory of the projection of the 

 different segments of the retina on different areas of his visual 

 sphere in dogs. According to this theory the central area A' 

 corresponds with the macula lutea, or retinal area of distinct 

 vision of the eye of the opposte side ; the more external portion of 

 A with the outer segment of the retina on the same side ; the 

 more internal portion of A with the inner segment of the retina 

 on the opposite side : the anterior half of the visual sphere is 

 related to the upper halves of the two retinae, and its posterior 

 half to the lower halves of both retinae. According to Munk, 

 therefore, it is possible in dogs to produce blindness of any sector 

 of each retina by extirpating the corresponding cortical area in 

 the visual sphere. This partial blindness will be permanent, just 

 as the total blindness is permanent after complete extirpation 

 of both visual spheres. He sought to apply the same theory to 

 monkeys, but admitted that his attempts were not conclusive. 



Undoubtedly if this theory of the projection of the retina on 

 the visual sphere of dogs had been founded on reliable experi- 

 mental facts, it would constitute the finest discovery in the physio- 

 logy of the cerebral cortex. But the subsequent researches of 

 Loeb, Goltz, and particularly of Luciani and Seppilli, who methodic- 

 ally re-tested Munk's theory, failed to substantiate it. 



It is certain from our own experiments with Seppilli (1885) 

 that obvious visual disorders occur in dogs not only after extirpa- 

 tion of the occipital lobe, but also after removing any other 

 extensive portion of the cortex, including the frontal lobes, that is, 

 the region furthest from Munk's visual sphere. This agrees with 

 the previous experiments of Goltz, Luciani and Tamburini, Hitzig, 

 Lautenbach, and others. But on closer consideration of the effect 

 on the visual function of destruction of the different portions of 

 the brain, there is seen to be an important difference : the visual 

 disorders that result from destruction of the frontal and temporal 

 lobes are transitory, while those that follow removal of the 

 occipital and parietal lobes are permanent the former do not 

 appear unless the frontal or temporal area destroyed is consider- 

 able the latter can easily be seen when only a small portion of 

 the cortex of the parieto-occipital lobes is removed. This fact 

 shows plainly that the localisation of the visual centre of the dog 

 in the cortex of the occipital lobe is mere speculation. Un- 

 doubtedly the cortex of the parietal lobe also forms an integral 

 part of this centre, which must spread even beyond its limits, 

 though it is not possible to determine the exact boundaries. 



