Elements of human physiology (1907) Elements of human physiology elementsofhumanp05star Year: 1907 NERVE-FIBRES (CONDUCTING TISSUES) 179 The E^ffect of Injury—The Bitter-VaUi Lato The irritability of the nerve of a muscle-nerve preparation is not equal in all parts of its course, but is greater at the upper end, probably in consequence of the proximity of the cross-section. Some time after a motor nerve is divided, the increased irritability at the upper end gives way to a decreased irrita- bility, and this decrease goes on till the nerve is no longer excitable. The diminution in excitability
Elements of human physiology (1907) Elements of human physiology elementsofhumanp05star Year: 1907 NERVE-FIBRES (CONDUCTING TISSUES) 179 The E^ffect of Injury—The Bitter-VaUi Lato The irritability of the nerve of a muscle-nerve preparation is not equal in all parts of its course, but is greater at the upper end, probably in consequence of the proximity of the cross-section. Some time after a motor nerve is divided, the increased irritability at the upper end gives way to a decreased irrita- bility, and this decrease goes on till the nerve is no longer excitable. The diminution in excitability gradually extends Fio. 85. Tracing of muscle contractions to show effect of cooling a nerve on ity excitability. The lower line indicates the changes in temperature of the excited part of the nerve. The muscle responded only when the nerve was cooled, the stimulus becoming ineffectual when the nerve was warmed (Gotch). down the nerve-fibre, so that the part of the nerve nearest the muscle remains excitable the longest. This progressive change in the irritability of a nerve after section is spoken of as the Eitter-Valli law. It is soon followed by definite histo- logical changes in the nerve, which we shall describe later on (see Chap. XIV.). The Influence of Temperature The excitability of a nerve is, within certain limits, in- creased by cooling the nerve, and diminished by raising its temperature (Fig. 85).^ Thus, if a frog be cooled to 2° or 3° C. ' This is true for all stimuli except induction-shocks and extremely short galvanic currents (less than 0-005 sec), with which the irritability of nerve is increased by warming and diminished by cooling. The same appears to be true for ventricular cardiac muscle, but in the case of voluntary muscle the excitability for all forms of stimuli is increased by cooling. (Gotch.)
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