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wardly on account of that extensive crossing of the fibres from the left hemisphere to the right half of the body only, which is shown in Fig. 43, below the letter M. But the left-brainedness might exist and not show outwardly. This would happen wherever organs on both sides of the body could be governed by the left hemisphere; and just such a case seems offered by the vocal organs, in that highly delicate and special motor service which we call speech. Either hemisphere can innervate them bilaterally, just as either seems able to innervate bilaterally the muscles of the trunk, ribs, and diaphragm. Of the special movements of speech, however, it would appear (from these very facts of aphasia) that the left hemisphere in most persons habitually takes exclusive charge. With that hemisphere thrown out of gear, speech is undone; even though the opposite hemisphere still be there for the performance of less specialized acts, such as the various movements required in eating.

The visual centre is in the occipital lobes. This also is proved by all the three kinds of possible evidence. It seems that the fibres from the left halves of both retina go to the left hemisphere, those from the right half to the right hemisphere. The consequence is that when the right occipital lobe, for example, is injured, hemianopsia' results in both eyes, that is, both retina grow blind as to their right halves, and the patient loses the leftward half of his field of view. The diagram on p. 111 will make this matter clear (see Fig. 45).

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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 retina 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

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

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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.

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.

Mental Blindness.-A most interesting effect of cortical disorder is mental blindness. This consists not so much

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FIG. 46.-Fibres associating the cortical centres together. (Schematic, atues

Starr.)

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 woul

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.

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The centre for hearing is situated in man in the upper convolution of the temporal lobe (see the part marked Wernicke' in Fig. 44). The phenomena of aphasia show this. We studied motor aphasia a few pages back; we must now consider sensory aphasia. Our knowledge of aphasia has had three stages: we may talk of the period of Broca, the period of Wernicke, and the period of Charcot. What Broca's discovery was we have seen. Wernicke was the first to discriminate those cases in which the patient can not even understand speech from those in which he can understand, only not talk; and to ascribe the former condition to lesion of the temporal lobe. The condition in question is word-deafness, and the disease is auditory aphasia. The latest statistical survey of the subject is that by Dr. Allen Starr. In the seven cases of pure word-deafness which he has collected (cases in

which the patient could read, talk, and write, but not understand what was said to him), the lesion was limited to the first and second temporal convolutions in their posterior two thirds. The lesion (in right-handed, i.e. leftbrained, persons) is always on the left side, like the lesion in motor aphasia. Crude hearing would not be abolished even were the left centre for it utterly destroyed; the right centre would still provide for that. But the linguistic use of hearing appears bound up with the integrity of the left centre more or less exclusively. Here it must be that words heard enter into association with the things which they represent, on the one hand, and with the movements necessary for pronouncing them, on the other. In most of us (as Wernicke said) speech must go on from auditory cues; that is, our visual, tactile, and other ideas probably do not innervate our motor centres directly, but only after first arousing the mental sound of the words. This is the immediate stimulus to articulation; and where the possibility of this is abolished by the destruction of its usual channel in the left temporal lobe, the articulation must suffer. In the few cases in which the channel is abolished with no bad effect on speech we must suppose an idiosyncrasy. The patient must innervate his speech-organs either from the corresponding portion of the other hemisphere or directly from the centres of vision, touch, etc., without leaning on the auditory region. It is the minuter analysis of such individual differences as these which constitutes Charcot's contribution towards clearing up the subject.

Every namable thing has numerous properties, qualities, or aspects. In our minds the properties together with the name form an associated group. If different parts of the brain are severally concerned with the several properties, and a farther part with the hearing, and still another with the uttering, of the name, there must inevitably be brought about (through the law of association which we shall later study) such a connection amongst all these brainparts that the activity of any one of them will be likely t

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