76 PRACTICAL PHYSIOLOGY 



It will be seen that the height of the contractions, although increasing 

 at first, gradually falls off until at the end of about 200 contractions it 

 reaches a uniform level, which represents about 85 per cent, of the 

 original height and was then maintained with scarcely any alteration 

 for three-quarters of an hour. This curve, therefore, is identical in 

 general form with that obtained by the ergograph. Here again we see 

 an initial fall and then a constant level of contraction, representing 

 probably the equilibrium between two opposite processes, which must 

 in this case be affecting some part of the peripheral nerve and muscle. 

 The actual seat of this peripheral change is not absolutely certain (see 

 further Expts. in Chapter XXII. ). 



Now cut through the leg in the middle of the thigh, so as to destroy 

 the circulation through the gastrocnemius and continue the stimulation 

 (Fig. 71). It will be seen that the height of the contractions rapidly 

 and continuously decreases, and that at the end of about 320 contrac- 

 tions the muscle is no longer able to lift the lever off the after-loading 

 screw. 



CHAPTER XVII. (Advanced). 



THE RATE OF TRANSMISSION OF A NERVOUS IMPULSE. 



THE rate at which an impulse is transmitted along a nerve is important 

 because it throws some light upon the nature of the impulse. It travels 

 much more slowly than an ordinary electric current, and, although it is 

 accompanied by an electric change, it is something more complex. Its 

 rate of propagation is 27 metres per second (88J feet per sec.) in the 

 frog's sciatic nerve, and 60 metres per second (196 feet per sec.) in 

 the motor nerves of man. 



(a) In the Motor Nerves of the Frog. The following experiment 

 should be performed for the determination of the velocity of the nervous 

 impulse in the sciatic nerve of a frog : 



A recording drum is arranged with a ' striker ' for completing the 

 circuit of the primary current of the induction-coil. To the secondary 

 coil are attached two Du Bois keys in the manner shown in the diagram 

 (Fig. 73) ; from these pass two pairs of electrodes, one of which will be 

 applied to the upper portion of the nerve, the other to the lower 



