80 



EXPERIMENTAL PHYSIOLOGY 



in amount, but in that they start from different points. Calling the 

 electrode at which the current enters the muscle the anode, and that 

 at which it leaves, the kathode, it is found that on make of the current 

 the contraction starts from the kathode and thence spreads over the 

 muscle, but that on break the contraction starts from the anode. 

 This very important point in the response of muscle to an electrical 

 current can be shown by the following experiments : — 



Experiment 6. — Dissect out a sartorius and fix it to record its move- 

 ments as shown in fig. 71. The muscle is lightly clipped between two 

 pieces of cork c and d at a point near its tibial end. A fine thread is tied 

 round the tendon at that end and attached to a crank lever, l. Two pins are 

 passed through the corks c d, and by these the muscle is fixed to the myo- 

 graph plate. The remaining longer piece of the muscle is connected to two 

 electrodes (unpolarisable, see p. 83) a and b, which are connected to a con- 

 stant current through a Pohl's commutator with cross wires, so that the 

 direction of the current may be reversed. The muscle is clamped so tightly 

 by the corks that it prevents any movement at a or b pulling on the piece of 

 sartorius s to the left of the cork clamp, and so moving the lever. It is not, 

 however, so tightly clamped as to prevent a wave of contraction passing 

 across from the piece s 1 to the piece s. If now a contraction start at one 

 instant from a, and passes as a wave along the muscle to s, a longer interval 

 will elapse before s begins to contract and raises the lever than if the con- 

 traction started at b and had only a short piece of muscle to travel along 



before it reached s. The 

 experiment now consists 

 in measuring the latent 

 periods of four curves : 

 two, one at make, the 

 other at break of a con- 

 stant current when the 

 current passes from a to 

 b, i.e. when the anode is 

 at a, and two when the 

 current travels from b to 

 a, i.e. when the anode is 

 at b. To record the in- 

 stants of opening and closing the current a signal included in the primary 

 circuit is arranged to record its movements directly beneath the myograph 

 lever. This does not give us an absolutely accurate measurement of the 

 latent period because there is a latency in the signal ; but as this is the same 

 in all four cases this does not matter, for we only require to measure differ- 

 ences in the latent period. 



Fig. 71.— Method of studying Polar Excitation of 

 a Muscle. 



It will be found that make of the ascending current (anode at b) 

 and break of the descending current (anode at a) have practically the 

 same latent period, and both are greater than those on break of the 

 ascending current or make of the descending current, which in their 

 turn are practically equal in value. 



Hence it is argued that the contraction on make of an ascending 



