290 PHYSIOLOGY 



vessels at c and D, and is attached to the thread. F, I, and G are three non- 

 polarisable electrodes composed of porous 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 sensi- 

 tive plate. The whole apparatus, with the exception of the glass rod at H, 

 can be immersed in a water bath at any given temperature. Two records are 



FIG. 107. Curve of muscle-twitch obtained by foregoing method. 



(KEITH LUCAS.) 

 A = moment of excitation. B = movement of muscle, c = time-marker. 



taken with the whole apparatus, first stimulating at c, and secondly stimu- 

 lating 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 18 C. or to 

 any desired temperature. 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 muscular 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 demon- 

 strate any phenomena of fatigue in the nerve-trunk.* This fact can 

 * Unless it be asphyxiated by total deprivation of oxygen. 



