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.[21]This list shows us how extraordinarily complex is the network of relations into which the auditory organ isbrought by its central paths.Apart from its twofold crossed and uncrossed connexion with the cerebral cortex,the following facts should be noted as of especial significance.First, there is a reflex path connecting theacoustic centres with the points of origin of muscular nerves, and among them with the centres for themovements of articulation and for the movements of the eyes, which latter are extremely important in thespatial orientation of the body.Secondly, we find that the conduction system, like that of the olfactory nerve,includes centrifugal paths, whose office is, perhaps, to transmit the excitations of the auditory organ of theopposite side, or other sensory excitations that find their nodal points in the mesencephalic region, in theform of concomitant sensation.We remark, in conclusion, that the acoustic nerve proper, which comes from the cochlea, is connected over apart of its peripheral course with the nerve that comes from the vestibule and canals.This, the vestibularnerve, is a branch of the eighth cranial, and is commonly accounted, like the cochlear, to the acoustic nerve.In its central course, however, it appears to follow a different road.It passes through special nidal structures,and finally, as its secondary degenerations prove, terminates in separate areas of the cerebral cortex.[22](d) Conduction Paths of the Optic NerveThe principal difference between the optic and acoustic conductions is that the optic surface itself, like theolfactory surface, is an outlying portion of the central organ, displaced to the periphery of the body.It isnatural, therefore, that the optic fibres too, when they emerge from the retina, should at once appear, as by farthe great majority of them do, in the character of central nerve fibres.The cells that give visual sensation itsspecific quality, the rods and cones (S and Z Fig.78) usually termed, on this account, visual cells aresensory epithelia which, like the gustatory cells, are connected only by contact with the terminal fibrils of theoptic conduction.In the retinal layers that cover them are several strata of nerve cells, easily divisible by theirmarked differences of form into two main groups: the large multipolar ganglion cells (G2), which my beregarded, from the relations of their neurites and dendrites, as proximate points of departure for the opticconduction running centripetally from the retina to the brain; and bipolar ganglion cells (G 1) to which may beadded stellate intercalary cells, found far forward in the neighbourhood of the elements S and Z, and notrepresented in the Figure.These last two classes constitute together a neurone territory, intervening betweenthe last termina: fibrils of the peripheral optic conduction and the large ganglion cells (G2), which may beconsidered as the extreme peripheral member of the centripetal optic conduction.Between its limits we find,further, terminal arborisations of neurites (e), derived not from cells of the retina itself but from more centralregions, probably from the pregemina, since these, as we shall see in a moment, form important nodalpoints in the optic conduction at large.There is thus a further point of resemblance between the outlyingcentral area represented in the retina and the olfactory surface: here as there, the structural relations indicatethe existence of a centrifugal secondary path, running alongside of the centripetal running alongside of thecentripetal principle path.[ 23]CHAPTER V.Course of the Paths of Nervous Conduction 104 Principles of Physiological PsychologyThe fibres collected in the optic nerve conduct, then, for the most part centripetally; though there is, in allprobability, a small admixture of centrifugal conductors [ Pobierz całość w formacie PDF ]
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.[21]This list shows us how extraordinarily complex is the network of relations into which the auditory organ isbrought by its central paths.Apart from its twofold crossed and uncrossed connexion with the cerebral cortex,the following facts should be noted as of especial significance.First, there is a reflex path connecting theacoustic centres with the points of origin of muscular nerves, and among them with the centres for themovements of articulation and for the movements of the eyes, which latter are extremely important in thespatial orientation of the body.Secondly, we find that the conduction system, like that of the olfactory nerve,includes centrifugal paths, whose office is, perhaps, to transmit the excitations of the auditory organ of theopposite side, or other sensory excitations that find their nodal points in the mesencephalic region, in theform of concomitant sensation.We remark, in conclusion, that the acoustic nerve proper, which comes from the cochlea, is connected over apart of its peripheral course with the nerve that comes from the vestibule and canals.This, the vestibularnerve, is a branch of the eighth cranial, and is commonly accounted, like the cochlear, to the acoustic nerve.In its central course, however, it appears to follow a different road.It passes through special nidal structures,and finally, as its secondary degenerations prove, terminates in separate areas of the cerebral cortex.[22](d) Conduction Paths of the Optic NerveThe principal difference between the optic and acoustic conductions is that the optic surface itself, like theolfactory surface, is an outlying portion of the central organ, displaced to the periphery of the body.It isnatural, therefore, that the optic fibres too, when they emerge from the retina, should at once appear, as by farthe great majority of them do, in the character of central nerve fibres.The cells that give visual sensation itsspecific quality, the rods and cones (S and Z Fig.78) usually termed, on this account, visual cells aresensory epithelia which, like the gustatory cells, are connected only by contact with the terminal fibrils of theoptic conduction.In the retinal layers that cover them are several strata of nerve cells, easily divisible by theirmarked differences of form into two main groups: the large multipolar ganglion cells (G2), which my beregarded, from the relations of their neurites and dendrites, as proximate points of departure for the opticconduction running centripetally from the retina to the brain; and bipolar ganglion cells (G 1) to which may beadded stellate intercalary cells, found far forward in the neighbourhood of the elements S and Z, and notrepresented in the Figure.These last two classes constitute together a neurone territory, intervening betweenthe last termina: fibrils of the peripheral optic conduction and the large ganglion cells (G2), which may beconsidered as the extreme peripheral member of the centripetal optic conduction.Between its limits we find,further, terminal arborisations of neurites (e), derived not from cells of the retina itself but from more centralregions, probably from the pregemina, since these, as we shall see in a moment, form important nodalpoints in the optic conduction at large.There is thus a further point of resemblance between the outlyingcentral area represented in the retina and the olfactory surface: here as there, the structural relations indicatethe existence of a centrifugal secondary path, running alongside of the centripetal running alongside of thecentripetal principle path.[ 23]CHAPTER V.Course of the Paths of Nervous Conduction 104 Principles of Physiological PsychologyThe fibres collected in the optic nerve conduct, then, for the most part centripetally; though there is, in allprobability, a small admixture of centrifugal conductors [ Pobierz całość w formacie PDF ]