392 STIMULATION OF NERVES. 



sec. IX.), and the double key with an induction coil. Arrange 

 for single opening induction shocks. 



By means of the double key, put the lower electrodes next 

 the muscle in connection with the secondary coil, and find 

 what strength of the current (what position of the secondary 

 coil) just falls short of causing a contraction. 



Then connect the upper electrodes with the secondary coil, 

 in place of the lower ones. Send through these a shock of 

 the same strength as that which sent through the lower elec- 

 trodes produced no contraction. A distinct contraction will 

 follow. 



Once more send the same shock through the lower elec- 

 trodes. There will be, as before, no contraction, or a very 

 slight one. 



The same stimulus produces, therefore, more effect when 

 applied to a point farther from the muscle. 



Obs. XVI. This is partly due to the section of the nerve 

 trunk above the higher electrodes. 



Having thoroughly destroyed the spinal cord of a frog, and 

 laid bare the sciatic nerve without dividing it, place a pair of 

 electrodes under the main sciatic trunk, send a feeble single 

 induction shock through them, and record the amount of con- 

 traction in the gastrocnemius, or determine the position of the 

 secondary coil, which gives a shock just falling short of the 

 strength required to cause a contraction. 



Divide the sciatic; nerve a little distance above the electrodes, 

 and determine, at intervals of 15 minutes, the contractions 

 which result from the application of the same stimulus as 

 before; or determine the position of the secondary coil for a 

 minimum stimulus. 



It will be found that the effect of the section is first to in- 

 Creole, and uftcnrurd* /<> f /"/;></// />//., the irritability of the 

 portions of the nerve lying immediately below the section. 



In the above observations, the student must make sure that 

 the electrodes are exactly similar, so that the differences which 

 come out are not due to any differences of resistance in the 

 two pair of electrodes or to the electrodes of one pair being 

 further apart from each other than those of the other, etc. 



For this purpose it will be as well, after a, series of observa- 

 tions, to exchange the electrodes, putting the one pair in the 

 former position of the other, and repeat the series. 



o/ys. XVII. On the sciatic nerve of a frog in which the 

 brain and spinal cord have been destroyed, and the heart 

 removed so as to stop the circulation, place three pair of 

 electrodes, one near the gastrocnemius, another close to the 

 central end of the nerve, and a third between the other two. 

 Divide the nerve above the upper pair. 



Arrange the preparation carefully in the moist chamber. 



