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.Certain sensory parts, e.g.the external auditory meatus and the larynx, are also pre-eminentlydisposed for concomitant sensation.[ 14] We can hardly explain these facts otherwise than by the hypothesisthat, on the one hand, certain preferred paths of connexion exist within the sensory conduction-paths, andthat, on the other, certain sensory areas (larynx, external auditory meatus) are peculiarly susceptible toco-excitation.As regards the conditions under which conduction takes place, it is clear that concomitantmovements and concomitant sensations both alike depend upon cross-conductions, that may be effected atdifferent heights in the myel, and that differ only in direction: the motor cross-conductions extending in alldirections, while the sensory, so far as we can tell, are almost exclusively unilateral, and for the most partfollow the direction from below upwards.For the rest, the concomitant sensations and concomitantmovements that have their ground in connexions of the myelic conduction-paths can never be certainlydiscriminated from those mediated by transference within the higher centres.This statement does not apply to transferences of the third kind, reflex connexions of sensory and motorpaths.The myelic reflexes may be observed for themselves alone, after the myel has been separated from thehigher central parts.The conclusion to be drawn from such observation is that branch-conduction of thereflexes is effected by a large number of conduction-paths, all of which are closely interconnected.Moderatestimulation of a circumscribed area of the skin is followed, at a certain mean degree of excitability, by areflex contraction in the muscle-group, and in that only, which is supplied by motor roots arising at the sameheight and on the same [p.163] side as the stimulated sensory fibres.If stimulus or irritability be increased,the excitation passes over, first of all, to the motor root fibres that leave the myel at the same height upon theopposite side of the body.Finally, if the increase be carried still farther, it spreads with growing intensity firstupward and then downward; so that in the last resort it involves the muscles of all parts of the body whichdraw their nerve-supply from myel and oblongata.It follows, then, that every sensory fibre is connected by abranch-conduction of the first order with the motor fibres arising on the same side and at the same height; byone of the second order, with the fibres issuing at the same height upon the opposite side; bybranch-conductions of the third order, with the fibres that leave the myel higher up; and, lastly, bybranch-conductions of the fourth order, with those that emerge lower down.[15] This law of the diffusion ofreflexes may, however, as we shall see in the following Chapter, be modified in two ways: by variation of theplace of application of the reflex stimulus, and by the simultaneous application of other sensory stimuli (cf.Chap.VI.§ 2).(c) Anatomical ResultsThe conclusions which we have reached by way of physiological experimentation regarding the course of theconduction-paths in the myel are in complete agreement with the morphological facts revealed byhistological examination of this organ.In particular, the arrangement of the nerve-cells and of thefibre-systems which take their origin from the cell-processes as shown in transverse and longitudinalsections, enables us to understand at once that every principal path is here accompanied by a large number ofsecondary paths, and that the most manifold connexions obtain between one line of conduction and another.CHAPTER V
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