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.Finally, we must mention (4) the plexus of stellate cells,which, while it plays but a small part in the centromotor regions, is never entirely wanting.This, if we mayjudge by the character that attaches to it in the occipital cortex, is to be regarded as the terminal station ofpaths of sensory conduction.Its position in the motor cortex is, as we have said, comparatively insignificant.It may be distinguished from the other constituents by its plexiform structure, the fibres running in allCHAPTER V.Course of the Paths of Nervous Conduction 120 Principles of Physiological Psychologypossible directions.The presence in the centromotor region of a formation which is characteristic of thesensory centres, may, perhaps, be taken to mean that this region is sensory too, as well as motor.Such aninterpretation would be in accordance with the fact that physiological experiment and pathologicalobservation place the centre for the general sense in this part of the cortex.We now turn, by way of contrast, to the 'visual' cortex, as a typical illustration of a pre-eminently sensoryregion.We are at once struck by the marked difference in the course of the fibres.Plexiform formations, withfibres running in all directions horizontally, therefore, as well as vertically and obliquely are verystrongly preponderant, while the longitudinal fibre masses characteristic of the terminal areas of thepyramidal paths find but scanty representation (Fig.96).These plexuses, C, are constituted of the large andsmall stellate cells, sending out processes in all directions, which appear conspicuously in the section of Fig.94; in all probability, they consist simply of interlocking neurones, of relatively limited range.Thecharacteristic systems of the motor area also appear, only in lesser numbers, in the visual cortex; just as theformations which we connect with the sensory functions are present, in some degree, throughout thecentromotor region.We notice, in particular, the longitudinal centrifugal fibres, connected with the pyramidalcells ( A).There are, further, the centripetal fibres, ascending to the cortex from deeper lying cell groups; and,lastly, the supposed association fibres with their intercalary cells (B).We must accordingly infer that thevisual cortex discharges centromotor, as well as centrosensory functions.The muscles that can be innervatedfrom it are, we may suppose, more especially the muscles of the eye, though it is possible that other motororgans, correlated with the ocular muscles, are also under its control.H.MUNK has observed movements ofthe eyes, in animals, as a result of stimulation of the visual cortex.[72]Such are the differences that obtain between the two main types of cortical structure, the sensory and themotor.Minor differences are found in the various parts of the motor cortex, and again between the visual areaand the other predominantly sensory regions.The former have already been discussed.As regards the latter,we notice that in the olfactory cortex of man the pyramidal cells are even rarer than in the visual; the smallerpyramidal cells are altogether wanting.The auditory cortex is characterised, on the other hand, by its greatwealth of stellate cells and by the extent of its sensory fibrillar reticula.In the 'association cortex,' finally,these plexuses become less conspicuous, and the granular layers, containing for the most part intercalary cellsof varying form, play the leading part.Putting all this together, we may sum up as follows the general outcome of investigation into the structuralpeculiarities of the cerebral cortex.Not only are the essential morphological elements the same, for alldivisions of the cortex, but their general arrangement also presents no really significant differences.At thesame time, there are several layers which, with their characteristic elements, attain to very different degreesof development according to the special functions of the various parts of the cortex [ Pobierz całość w formacie PDF ]