CONDITIONS AFFECTING A NERVOUS IMPULSE 



259 



FIG. 106. 



clay, containing saturated zinc sulphate, in which a zinc rod is immersed. If the current 

 is passed in at G and out at r the effective cathode is at the lower end of the constriction 

 c, and similarly if the current is passed in at I and out at G, the effective cathode is at 

 D. The tendon of the muscle A is attached by a thin glass rod H to a very light recording 

 lever, the movement of which is magnified by placing it in the focal plane of a projecting 

 eye-piece and recording its image on a moving sensitive plate. The whole apparatus, with 

 the exception of the glass rod at H, can be immersed in a water bath at any given tempera- 

 ture. Two records are taken with the whole apparatus, first stimulating at c, and 

 secondly stimulating at D. The difference between the latent periods in these two 

 cases is the time taken for the excitatory wave to travel from D to c. The rate of 

 propagation is similarly 

 recorded when the water 

 bath is raised to 18C. 

 or to any desired tempera- 

 ture. Since we are only 

 dealing with differences 

 in latent periods the effect 

 of the rise of temperature 

 on the latent period of the 

 muscle itself does not 

 affect the determinations. 



THE INFLUENCE 

 OF FATIGUE. In the 

 description of the 

 phenomena of mus- 

 cular fatigue* given in 

 the last chapter, it was 

 assumed that the 

 muscle was being 



excited directly. The same phenomena are observed when the muscle 

 is excited through its nerve, though in this case fatigue comes on much 

 more quickly. If, after the muscle has been excited in this way until 

 exhausted, it be excited directly, it will respond with a contraction nearly as 

 high as at the beginning of the experiment. We see therefore that the 

 nervous structures are more susceptible to the influences causing fatigue 

 than the muscle itself, and it can be shown that the weak point in the nerve- 

 muscle preparation is not the nerve, but the end-plates. In fact it is not 

 possible to demonstrate any phenomena of fatigue in the nerve-trunk.* 

 This fact can be shown in mammals by poisoning the animal with curare, and 

 then stimulating a motor nerve continuously while the animal is kept alive 

 by means of artificial respiration. As the effect of the curare on the end- 

 plates begins to wear off in consequence of its excretion, the muscles supplied 

 by the stimulated nerve enter into tetanus. The action of the curare may be 

 cut short at any time by the injection of salicylate of physostigmine, when 

 the muscles will at once begin to react to the excitation. 



The same fact may be shown on the excised nerve-muscle preparation 

 of the frog. The gastrocnemii of the two sides with the sciatic nerves are 

 dissected out, and an exciting circuit is so arranged that the interrupted 



* Unless it be asphyxiated by total deprivation of oxygen. 



Curve of muscle-twitch obtained by foregoing 



method. (KEITH LUCAS.) 



A = moment of excitation. B = movement of muscle, 

 c = time-marker. 



