Quite recently, both Schaefer and Munk, in studying the movements of the eyeball produced by galvanizing the visual cortex in monkeys and dogs, have found reason to plot out an analogous correspondence between the upper and lower portions of the retinæ and certain parts of the visual cortex. If both occipital lobes were destroyed, we should have double hemiopia, or, in other words, total blindness. In human hemiopic blindness there is insensibility to light on one half of the field of view, but
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.
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.
Where an object fails to be recognized by sight, it often happens that the patient will recognize and name it as soon as he touches it with his hand. This shows in an interesting way how numerous are the incoming paths which all end by running out of the brain through the channel of speech. The hand-path is open, though the eye-path be closed. When mental blindness is most complete, neither sight, touch, nor sound avails to steer the patient, and a sort of dementia which has been called asymbolia or apraxia is the result. The commonest articles are not understood. The patient will put his breeches on one shoulder and his hat upon the other, will bite into the soap and lay his shoes on the table, or take his food into his hand and throw it down again, not knowing what to do with it, etc. Such disorder can only come from extensive brain-injury.