FIG. 45.--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_. ] mental images of visible things remain. In _double_ hemiopia there is every reason to believe that not only the sensation of light must go, but that all memories and images of a visual order must be annihilated also. The man loses his visual 'ideas.' Only 'cortical' blindness can produce this effect on the ideas. Destruction of the retinæ or of the visual tracts anywhere between the cortex and the eyes impairs the retinal sensibility to light, but not the power of visual imagination. [Illustration: FIG. 46.--Fibres associating the cortical centres together. (Schematic, after Starr.)] =Mental Blindness.=--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 interpretable as _loss of associations_ between 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, printed letters of the alphabet, or words, signify both certain sounds and certain articulatory movements. But the connection between the articulating or auditory centres and those for sight being ruptured, we ought _a priori_ to expect that the sight of words would fail to awaken the idea of their sound, or of the movement for pronouncing them. We ought, in short, to have _alexia_, or inability to read: and this is just what we do have as a complication of _aphasic_ disease in many cases of extensive injury about the fronto-temporal regions.