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.It passes directly from thecortex to the motor arrangements in the cord, depending for its proper nutrition (as the facts ofdegeneration show) on the influence of the cortical cells, just as motor nerves depend for theirnutrition on that of the cells of the spinal cord.Electrical stimulation of this motor strand in anyaccessible part of its course has been shown in dogs to produce movements analogous to thosewhich excitement of the cortical surface calls forth.One of the most instructive proofs of motor localization in the cortex is that furnished by thedisease now called aphemia, or motor Aphasia.Motor aphasia is neither loss of voice norparalysis of the tongue or lips.The patient's voice is as strong as ever, and all the innervations ofhis hypoglossal and facial nerves, except those necessary for speaking, may go on perfectly well.He can laugh and cry, and even sing; but he either is unable to utter any words at all; or a fewmeaningless stock phrases form his only speech ; or else he speaks incoherently and confusedly,mispronounc-[p.38] ing, misplacing, and misusing his words in various degrees.Sometimes hisspeech is a mere broth of unintelligible syllables.In cases of pure motor aphasia the patientrecognizes his mistakes and suffers acutely from them.Get any book for free on: www.Abika.comTHE PRINCIPLES OF PSYCHOLOGY22Now whenever a patient dies in such acondition as this, and an examination of his brain ispermitted, it is found that [p.39] the lowest frontalgyrus (see Fig.11) is the seat of injury.Broca firstnoticed this fact in 1861, and since then the gyrushas gone by the name of Broca's convolution.The injury in right-handed people is found on the lefthemisphere, and in left-handed people on the righthemisphere.Most people, in fact, are left-brained, that is,all their delicate and specialized movements are handedover to the charge of the left hemisphere.The ordinaryright-handedness for such movements is only aconsequence of that fact, a consequence which showsoutwardly on account of that extensive decussation of thefibres whereby most of those from the left hemisphere passto the right half of the body only.But the left-brainednessmight exist in equal measure and not show outwardly.Thiswould happen wherever organs on both sides of the bodycould be governed by the left hemisphere; and just such acase seems offered by the vocal organs, in that highlydelicate and special motor service which we call speech.Either hemisphere can innervate them bilaterally, just aseither seems able to innervate bilaterally the muscles of thetrunk, ribs, and diaphragm.Of the special movements ofspeech, how-[p.40] ever, it would appear (from the facts ofaphasia) that the left hemisphere in most persons habituallytakes exclusive charge.With that hemisphere thrown out ofgear, speech is undone; even though the oppositehemisphere still be there for the performance of lessspecialized acts, such as the various movements required in eating.It will be noticed that Broca's region is homologous with the parts ascertained to producemovements of the lips, tongue, and larynx when excited by electric currents in apes (cf.Fig.6, p.34).The evidence is therefore as complete as it well can be that the motor incitations to theseorgans leave the brain by the lower frontal region.Victims of motor aphasia generally have other disorders.One which interests us in thisconnection has been called agraphia: they have lost the power to write.They can read writingand understand it; but either cannot use the pen at all or make egregious mistakes with it.Theseat of the lesion here is less well determined, owing to an insufficient number of good cases toconclude from.[12] There is no doubt, however, that it is (in right-handed people) on the leftside, and little doubt that it consists of elements of the hand-and-arm region specialized for thatservice.The symptom may exist when there is little or no disability in the hand for other uses.Ifit does not get well, the patient usually educates his right hemisphere, i.e.learns to write with hisleft hand.In other cases of which we shall say more a few pages later on, the patient can writeGet any book for free on: www.Abika.comTHE PRINCIPLES OF PSYCHOLOGY23both spontaneously and at dictation, but cannot read even what he has himself written! All thesephenomena are now quite clearly explained by separate brain-centres for the various feelings andmovements and tracts for associating these together.But their minute discussion belongs tomedicine rather than to general psychology, and I can only use them here to illustrate theprinciples of motor localization [ Pobierz całość w formacie PDF ]