226 PRACTICAL PHYSIOLOGY. [XLIII. 



(d.) Close the key ot the polarising circuit, i.e., remove the block. The 

 muscle responds at once. Therefore the loss of excitability or seat of exhaus- 

 tion is not in the nerve (Bernstein}. Where is it, then ? It must lie primarily 

 somewhere between the nerve and muscle, i.e., it is in the end-plates, or where 

 nerve joins muscle. Moreover, Bowditch has shown that the sciatic nerve of 

 a curarised cat may be stimulated for hours, there being no muscular 

 response, but as soon as the effect of curare, which is known to paralyse the 

 nerve-terminals in striped muscle, passes off, the muscles of the foot respond. 



C. The two results of B and C may be combined thus : 



(a.) Dissect out two nerve-muscle preparations (A and B) from a frog, 

 clamp both femurs in one clamp, and attach straw nags of different colours 

 to both legs (fig. 114). Lay both nerves over a pair of Du Bois electrodes. 

 Cover them, keep them moist. 



(b. ) Attach the electrode wires to two of the binding screws of the commu- 

 tator without cross-bars, turning the handle, so that the current can be passed 

 through both nerves when desired. 



(c.) To the nerve of B. between the Du Bois electrodes and the muscle, 

 apply a "polarising current" with its - pole next the muscle. 



(d.) Pass an interrupted current through both nerves ; A will become tetanic 

 while B remains quiescent ; the impulse cannot pass because of the "block" 

 produced by the "polarising current." 



(.) Continue to stimulate the nerves until A ceases to respond. Break 

 the polarising current, i.e., remove the block on B ; B becomes tetanic. 



As both nerves have been equally stimulated, both are equally fatigued or 

 non-fatigued. As B becomes tetanic, the seat of the fatigue is' not in the 

 nerve- trunk. 



As in A the seat of fatigue was not in the muscle, and as B shows that 

 nerve-fibres practically do not manifest the signs of fatigue, it would seem 

 that its seat must be somewhere between muscle and nerve, in all probability 

 in the end-plates. 



LESSON XLIII. 



MUSCLE WAVE MUSCLE THICKENING WILD'S 

 APPARATUS. 



1. This is best done by the method originally used by v. Bezold, 

 and modified in a simple form by Biedermann. A muscle with 

 parallel fibres preferably a sartorius is fixed a little to one side of 

 the middle line in a cork clamp so that the direct transference of 

 the change of muscle form, but not the excitation process in the 

 muscle, is prevented from passing, i.e., one part of the muscle is 

 stimulated while the other part records. 



(a.) Arrange an induction machine in connection with a com- 

 mutator without cross-bars and two pairs of thin wires, so as to be 

 able to send a single maximum break shock through either pair of 

 wires as in the curare experiment (Lesson XXXIII.). Let the 

 primary current be broken by the automatic drum key. Arrange 



