136 Visuo-Sensory and Visuo- Psychic Areas [CHAP. 



posterior cerebral artery 1 , but neither the distribution of the whole of this vessel nor of 

 any given branch or branches exactly corresponds to the distribution of what we believe 

 to be the visual area. In other words, when the occipital artery is occluded by thrombosis 

 or embolism, other parts in addition to the visual cortex are obliterated ; not only this, but 

 when the same artery or any of its branches, and particularly the calcarine branch, is oc- 

 cluded, incalculable damage is done to the occipito-thalamic and other fibres radiating to 

 all parts of the occipital cortex. The truth of this is emphasised by von Monakow, and 

 is illustrated fully by a number of specimens which I have collected from patients who have 

 died at Rainhill. Accordingly we are forced to conclude that the lesions produced by arterial 

 occlusion, cerebral tumour, abscess formation, haemorrhage, or by any other destructive agent, 

 are of too gross a nature to allow of precise localisation, and the corollary follows that as 

 the result of a natural process we can never hope to obtain an example of an obliterative 

 destruction confined exactly to the visual area and involving the cortex only, for such a 

 lesion is an anatomical impossibility. 



Although faced by such difficulties it is gratifying to find that, as the number of 

 reported cases increases and positive and negative evidence is accumulated and carefully 

 assimilated, the limitations of the area become gradually more restricted and definite, and 

 so experimental, clinical, and anatomical differences progressively diminish. 



Attempting a brief summary of the data adduced from the clinico-pathological side, we 

 find that there is considerable agreement in opinion on the point that a lasting hemiaiiopic 

 condition is produced most readily by a lesion in the neighbourhood of the calcarine fissure ; 

 thus Henschen, basing his conclusions on forty cases of hemianopsia collected in Upsala, 

 states that a lesion involving the cortex of the calcarine fissure particularly of its anterior 

 two- thirds alone will suffice to bring about lasting hemianopsia, and that the remainder of 

 the occipital lobe is of no consequence in so far as the production of such a visual defect 

 is concerned. 



Huguenin, who wrote before Henschen, also confined the visual area within similar narrow 

 limits, but apparently attached more importance to the posterior than the anterior calcarine 

 cortex. 



Hun published an important case in which destruction of the cuneus bordering on the 

 calcarine fissure produced a hemiaiiopic defect limited to the upper right quadrant of 

 each retina, and accepting this as a direct example of cause and effect, he assumed that 

 the fibres from the lower right quadrant must proceed to the cortex of the lingual lobule, 

 immediately underlying the posterior limb of the calcarine fissure. 



Cases of bilateral hemianopsia seem to be decidedly uncommon, and apparently only 

 five are to be found in the literature (Forster, Berger, Schweigger, Kaesterman, von Monakow 



1 Figures correctly indicating the occipital distribution of the posterior cerebral artery are given in von Mouakow's 

 admirable text-book and in Boltou's paper, and further injections and dissections carried out by Doctor H. A. Eobinsou 

 and myself show that there is nothing to add to the description given by the observers named. Like all other 

 arteries these are subject to variations and irregularities, but the following may be looked upon as the commonest 

 arrangement : after giving off an ascending twig to the splenium and a descending twig to the lingual lobule the 

 occipital artery (Duret) dips down into the stem of the calcarine fissure and divides into two branches, which have 

 received the names parieto-occipital and calcariue respectively. The former ascends the fissure after which it is 

 named; it gives off, first, a twig called the cuneal for the supply of the cuneus, secondly, twigs for the bounding 

 convolutions of the fissure, and, finally, it ends by ramifying on the convexity of the hemisphere. The calcarine artery 

 follows the calcarine fissure, supplying both its walls and ending on the posterior surface of the occipital lobe. 



