92 AN AMERICAN TENT-BOOK OF PHYSIOLOGY. 



to the excited area. When a muscle is dying, the rate of conduction as well 

 as the rapidity of contraction is lessened. The muscles of warm-blooded ani- 

 mals exhibit more striking differences than those of cold-blooded, but both are 

 similarly affected. 1 fa dying muscle be mechanically stimulated, as by a direct 

 blow, a localized swelling develops at the place; and if the muscle be stroked 

 with a dull instrument, a wave of contraction maybe seen to follow the instru- 

 ment, the contraction being quite strictly limited to the excited area, so that 

 one can write on the muscle. The strict localization of the contraction to the 

 irritated parts makes it evident that the nerves take no part in it, hence Schiff 

 called it an idio-muscular contraction, in distinction from the normal neuro- 

 muscular contraction. In the dying nerve as in the dying muscle the rate of 

 transmission is found to be slowed. 



(6) Effect of Mechanical Conditions. — The effect of pressure to lessen the 

 conduction-power of nerves is one which everyone has had occasion to demon- 

 strate on himself. For example, if pressure be brought to bear on the ulnar 

 nerve where it crosses the elbow, the region supplied by the nerve becomes numb, 

 "goes to sleep," so to speak. It is noticeable that only a slightly greater 

 effort is required to move the muscles, at a time when no sensations are received 

 from the hand. For some unexplained reason the sensory nerve-fibres appear 

 to be less resistant than the motor. Gradually applied pressure may paralyze 

 the nerve without exciting it, but on the removal of the pressure the recovery 

 of function of the sensory fibres is accompanied by excitation processes, which 

 are felt as pricking sensations referred to the region supplied by the nerve. The 

 exact reason of the loss of functional power caused by pressure which is insuf- 

 ficient to produce permanent injury is not altogether clear. Stretching a nerve 

 may act to lessen, and if severe destroy, conductivity. It is in one sense another 

 way of applying pressure, since the calibre of the sheath is lessened and through 

 the fluids pressure is brought to bear on the axis-cylinder. Of course, if the 

 stretching were excessive, the nerve-fibres would be ruptured and degenerate. 



Whether stretching can alter the rate of conduction in nerves is not known. 

 Apparently it does not do so in muscles, although because of the greater 

 length of the muscle it appears to do so. 1 



(c) Effect of Temperature on Conduction. — Helmholtz and Baxt found that 

 by cooling motor nerves they could lower the rate of conduction, and by heat- 

 ing them they could increase it even more markedly. By altering the tem- 

 perature of the motor nerves of man, they observed rates varying from 30 to 

 90 meters per second. The rate of the motor nerves of other animals is like- 

 wise greatly altered by heat and cold. This is true of the invertebrates as well 

 ;i~ the vertebrates; the rate in the nerves of the claw-muscles of the lobster, 

 for example, changes from 6 to 12 meters per second as the temperature is 

 varied from 10° to 20° C. Sensory nerve-fibres are similarly influenced by 

 temperature. Oehl 2 found by cooling and heating the nerves of men, varia- 

 tions of from 30 to 25 meters per second on cooling, and from 30 to 50 meters 



1 Schenck : Pfiuger's Archiv, 1896, Bd. 64, S. 179. 

 'Oehl: Archives italiennes de Bioloyie, 1895, xiiv. p. 231. 



