Hemiopia may also result from lesion in other parts, especially the neighboring angular and supra-marginal gyri, and it may accompany extensive injury in the motor region of the cortex. In these cases it seems probable that it is due to an _actio in distans_, probably to the interruption of fibres proceeding from the occipital lobe. There seem to be a few cases on record where there was injury to the occipital lobes without visual defect. Ferrier has collected as many as possible to prove his localization in the angular gyrus.[23] A strict application of logical principles would make one of these cases outweigh 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 'decussation of the pyramids,' nor any more usual pathological 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.[24] If, in such a case as 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_ below [Fig. 15], 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.[25] [Illustration: FIG. 15.--Scheme of the mechanism of vision, after Seguin. The _cuneus_ convolution (_Cu_) of the right occipital lobe is supposed to be injured, and all the parts which lead to it are darkly shaded to show that they fail to exert their function. _F. O._ are the intra-hemispheric optical fibres. _P. O. C._ is the region of the lower optic centres (corpora geniculata and quadrigemina). _T. O. D._ is the right optic tract; _C_, the chiasma; _F. L. D._ are the fibres going to the lateral or temporal half _T_ of the right retina; and _F. C. S._ are those going to the central or nasal half of the left retina. _O. D._ is the right, and _O. S._ the left eyeball. The rightward half of each is therefore blind: in other words, the right nasal field, _R. N. F._, and the left temporal field _L. T. F._, have become invisible to the subject with the lesion at _Cu_.] * * * * *