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.But the weakest reflexcontractions are always noticeable by their long duration, and the strongest by the length of their latentperiod, even when we compare the former with the strongest and the latter with the weakest directcontractions.[20] It is clear, now, that the time which the stimulation requires to pass from a sensory into amotor root is given by the interval separating the beginnings of the two contractions, the direct and the reflex.The nerve-roots are so short, that the portion of this interval taken up by the peripheral conduction may beconsidered negligible; and we may accordingly designate the interval as a whole, the reflex time.Todetermine it, we must since the latent period is dependent upon the intensity of the stimuli have recourseonce more, as we did in our measurement of the rate of propagation in the nerves, to experiments in whichthe muscle-nerves are of the same height.This presupposed, we may proceed to investigate the reflex time under various conditions.The simplest caseis shown in Fig.31, where the stimulation is transferred from a sensory root to a motor root belonging to thesame nerve-trunk: we may term this the case of same-sided reflex excitation.Next in order comes thepropagation of stimulus from a sensory root to a motor root which leaves the myel at the same height butupon the opposite side: we term this the case of crossed reflex excitation.In the third place, we may havepropagation along the length of the myel, which we may call the longitudinal conduction of reflexes; as, e.g.,in the transference from the sensory root of a nerve of the arm to the motor root of a nerve of the lowerextremities.In no one of these three cases is the reflex time sensibly dependent upon the intensity of theexcitations.It is, as might have been predicted, relatively shortest for same-sided reflex excitation, whereunder normal circumstances it amounts to 0.008 to 0.015 sec.It is, however, as one would be less likely toexpect, relatively greater with crossed than with longitudinal conduction.Thus, if we compare the crossedwith the same-sided reflex, there is an average difference to the disadvantage of the former of some 0.004sec.If we then compare the reflex released in the thigh by stimulation of the root of a sensory nerve of thearm with the same-sided reflex, the difference between the two times is as a rule somewhat smaller.[ 21]Since the path travelled by the stimulation in the latter case is at least six to eight times as long as thattraversed in the former, it is evident that the retardation in crossed conduction is much more serious than it isin longitudinal conduction.An explanation is, without any doubt, to be found in the fact that longitudinalconduction (as we shall see presently, in Chapter V., when we come to discuss the morphology of the myel)is sub- served for the most part by the fibres of the white substance, while crossed conduction must bemediated almost exclusively by the cell-reticulum of the grey substance.We have, then, in the results of thisset of experiments, a confirmation of the inference, already suggested as probable by the long duration of thereflex time, that the central elements offer incomparably more resistance than the nerve-fibres to theprogress of an excitation.The same conclusion may be drawn from the further fact that a retardation ofconduction, amounting on the average to 0.003 sec., occurs in the spinal ganglia of the frog, and again fromthe related observation that the sensory nerve roots are more irritable than the nerve-fibres below the spinalganglia.It is noteworthy, in connexion with this latter result, that the ramifications of the sensory nerves inthe skin are, in their turn, more easily excitable than the nerve-branches that run to the skin.Just, then, asthere are mechanisms in the spinal ganglia which diminish the irritability of the incoming nerves, so mustthere he mechanisms in the skin which discharge a precisely opposite function [ Pobierz całość w formacie PDF ]
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.But the weakest reflexcontractions are always noticeable by their long duration, and the strongest by the length of their latentperiod, even when we compare the former with the strongest and the latter with the weakest directcontractions.[20] It is clear, now, that the time which the stimulation requires to pass from a sensory into amotor root is given by the interval separating the beginnings of the two contractions, the direct and the reflex.The nerve-roots are so short, that the portion of this interval taken up by the peripheral conduction may beconsidered negligible; and we may accordingly designate the interval as a whole, the reflex time.Todetermine it, we must since the latent period is dependent upon the intensity of the stimuli have recourseonce more, as we did in our measurement of the rate of propagation in the nerves, to experiments in whichthe muscle-nerves are of the same height.This presupposed, we may proceed to investigate the reflex time under various conditions.The simplest caseis shown in Fig.31, where the stimulation is transferred from a sensory root to a motor root belonging to thesame nerve-trunk: we may term this the case of same-sided reflex excitation.Next in order comes thepropagation of stimulus from a sensory root to a motor root which leaves the myel at the same height butupon the opposite side: we term this the case of crossed reflex excitation.In the third place, we may havepropagation along the length of the myel, which we may call the longitudinal conduction of reflexes; as, e.g.,in the transference from the sensory root of a nerve of the arm to the motor root of a nerve of the lowerextremities.In no one of these three cases is the reflex time sensibly dependent upon the intensity of theexcitations.It is, as might have been predicted, relatively shortest for same-sided reflex excitation, whereunder normal circumstances it amounts to 0.008 to 0.015 sec.It is, however, as one would be less likely toexpect, relatively greater with crossed than with longitudinal conduction.Thus, if we compare the crossedwith the same-sided reflex, there is an average difference to the disadvantage of the former of some 0.004sec.If we then compare the reflex released in the thigh by stimulation of the root of a sensory nerve of thearm with the same-sided reflex, the difference between the two times is as a rule somewhat smaller.[ 21]Since the path travelled by the stimulation in the latter case is at least six to eight times as long as thattraversed in the former, it is evident that the retardation in crossed conduction is much more serious than it isin longitudinal conduction.An explanation is, without any doubt, to be found in the fact that longitudinalconduction (as we shall see presently, in Chapter V., when we come to discuss the morphology of the myel)is sub- served for the most part by the fibres of the white substance, while crossed conduction must bemediated almost exclusively by the cell-reticulum of the grey substance.We have, then, in the results of thisset of experiments, a confirmation of the inference, already suggested as probable by the long duration of thereflex time, that the central elements offer incomparably more resistance than the nerve-fibres to theprogress of an excitation.The same conclusion may be drawn from the further fact that a retardation ofconduction, amounting on the average to 0.003 sec., occurs in the spinal ganglia of the frog, and again fromthe related observation that the sensory nerve roots are more irritable than the nerve-fibres below the spinalganglia.It is noteworthy, in connexion with this latter result, that the ramifications of the sensory nerves inthe skin are, in their turn, more easily excitable than the nerve-branches that run to the skin.Just, then, asthere are mechanisms in the spinal ganglia which diminish the irritability of the incoming nerves, so mustthere he mechanisms in the skin which discharge a precisely opposite function [ Pobierz całość w formacie PDF ]