48 PSYCHOLOGY. 



fibres proceeding from the occipital lobe. There seem to 

 be a few cases on record where there was injury to the 

 occipital lobes Avithout visual defect. Ferrier has collected 

 as many as possible to prove his localization in the angular 

 gyrus.* A strict application of logical principles would make 

 one of these cases outAveigh one hundred contrary ones. And 

 yet, remembering how imperfect observations may be, and 

 how individual brains may vary, it would certainly be rash for 

 their sake to throw away the enormous amount of positive 

 evidence for the occipital lobes. Individual variability is 

 always a possible explanation of an anomalous case. There 

 is no more prominent anatomical fact than that of the 'de- 

 cussation of the pyramids,' nor any more usual pathologi- 

 cal fact than its consequence, that left-handed hemorrhages 

 into the motor region produce right-handed paralyses. 

 And yet the decussation is variable in amount, and seems 

 sometimes to be absent altogether. t If, in such a case a& 

 this last, the left brain were to become the seat of apoplexy, 

 the left and not the right half of the body would be the 

 one to suffer paralysis. 



The schema on the opposite page, copied from Dr, 

 Seguin, expresses, on the whole, the probable truth about the 

 regions concerned in vision. Not the entire occipital lobes, 

 but the so-called cunei, and the first convolutions, are the 

 cortical parts most intimately concerned. Nothnagel agrees 

 with Seguin in this limitation of the essential tracts.:}: 



A most interesting effect of cortical disorder is mental 

 blindness. This consists not so much in insensibility to 

 optical impressions, as in inability to understand them. 

 Psychologically it is interpre table as loss of associations be- 

 tween optical sensations and what they signify ; and any 

 interruption of the paths between the optic centres and the 

 centres for other ideas ought to bring it about. Thus, 



* Localizatiou of Cerebral Disease (1878), pp. 117-8. 



f For cases see Flechsig : Die Leitungsbahnen iu Gehiru ii. Riickenmark 

 (Leipzig, 1876), pp. 112, 272; E.vner's Uutersiichuugen, etc., p. 83 ; Ferrier s 

 Localization, etc., p. 11; Fran^ois-Franck's Cerveau Moteur, p. 63, note. 



X E. C. Seguin : Hemianopsia of Cerebral Origin, iu Journal of Nervous 

 and Mental Disease, vol. xiii. p. 30. Notlinagel und Naunyu : Ueber die 

 Localization der Gebirnkrankheiteu (Wiesbaden, 1887), p. 10. 



