760 



THE CEREBRAL CORTEX. 



colour vision upon the homonymous side of the two retinae may precede the 

 hemiopia ; but it has not yet been determined that colour vision is dependent 

 upon the integrity of a different part of the cortex than appreciation of form. 



Occasionally hallucinations — in perfectly sane subjects — are present in 

 hemiopia, the sensations originating on the blind side of the brain. In these 

 cases either the lesion producing hemiopia is subcortical, or if cortical must be 

 assumed to be incomplete. Such unilateral hallucinations, with or without 

 hemiopia, are also sometimes seen in attacks of hemicrania. 1 



Cases of localised bilateral lesions of the internal surface of the occipital lobes, 

 producing complete blindness, have been published by various observers. 2 In 

 some of these cases both lobes were affected simultaneously, in others first one 

 lobe was affected, and a condition of hemiopia was produced, and then after an 

 interval of time the other lobe, the condition being converted into double 

 hemiopia or total blindness. In Forster's case central vision was still possible, 

 although of a very imperfect character; it was found by Sachs, who made the 

 examination of the brain, that a small portion of the grey matter of the cortex 

 bounding the right calcarine fissure had not been involved in the destruction ; 

 but whether this was sufficient to account for the persistence of some central 

 vision, cannot be definitely affirmed, especially since there have been several 

 other cases of double hemianopsia in which some amount of central vision has 

 persisted. 3 In this patient colour vision was absent, although the appreciation 

 of form persisted, as shown by the fact that he could recognise common objects 

 and even read and write. There was, however, a marked loss of memory of 

 locality, and lack of power to appreciate the relative positions of objects, such as 

 the position of countries and places on a map, the position and direction of the 

 paths in a garden, or even the position of furniture and accustomed objects in 

 his own room ; although there was no such loss of memory of position of objects 

 the appreciation of which was ordinarily determined by the sense of touch, 

 such as parts of his own person, clothing, etc. In fact, although this patient 

 still had some central vision, he was able to find his way about with far less 

 facility than persons who are totally blind from destruction of the peripheral 

 visual organs, and whose other senses have become sharpened by disuse of the 

 visual sense. An animal (monkey, dog) rendered blind by removal of the 

 occipital lobes is unable for an indefinite time to find his way freely about even 

 the most familiar place without bumping against objects, and shows a great 

 disinclination to move about ; whereas it is a familiar fact that an animal which is 

 blind in consequence of a peripheral lesion, speedily accommodates itself to the 

 altered circumstances in which it is situated, and has no difficulty in finding its way 

 about places with which it is familiar (H. Munk). Sachs 4 attempts to explain 

 some of the phenomena which were observed in Forster's case, by supposing 

 that the appreciation of the quality of light coming from objects is effected in a 

 manner different from that of the form and relative position of objects, that 

 the perception of form and relative position involves motion of the eyes, and is 

 associated with sensations produced by such motion, and that this perception is 

 localised in other parts of the occipital lobes than those concerned in the 

 appreciation of colour. Wilbrand finds it necessary, in consideration of various 

 phenomena which have been observed clinically, to assume three centres of 

 visualisation — one for the mere appreciation ^of luminosity, one for appreciation 

 of colour, and one for appreciation of form and relative position of objects 

 (space) ; he supposes that these may all be situated in the same region of the 

 brain, but in different layers of the cortex. Holden 5 noticed that partial loss of 



1 J. Soury, article "Cerveau," Richet's "Diet, de physiol.," p. 919. 



2 Bouvertt, Rev. gin. d'opht., Paris, 30th November 1897 ; Oulmont, Gaz. held, de mid., 

 Paris, 1889. p. 607; Chauffard, Rev. de vied., Paris, 1888, p. 131; Forster, Arch. f. 

 Ophfh., Leipzig 1890, Bd. i. S. 94; Sachs, Arb. a. d. psych. Klin, zu Brcslau, Leipzig, 

 1895, S. 58 ; Dejerine and Vialet, Compt. rend. Soc. de biol., Paris, 1893, p. 983. 



3 See Soury, article "Cerveau," loc. cit., p. 949. 4 Loc. cit. 

 5 Arch./. Ophth., Leipzig, 1896. 



