202 MUSCLE AND NERVE. 



the nerve is suddenly pulled or struck, it is thrown into 

 action. Try these forms of stimuli last, upon a nerve which 

 has been in use for some other experiment. 



(6) Thermal stimuli. Heat a stout copper wire in a flame, 

 and apply it suddenly when very hot to the nerve. 



(c) Chemical stimuli. Apply a drop of saturated chloride 

 of sodium to either the end or to some other part of a 

 nerve, its muscle will commence twitching and will soon pass 

 into a continued contraction (salt tetanus). The contractions 

 are, however, not completely fused. 



Changes in the excitability of a nerve when dying 1 . Lay 



beneath the whole length of the sciatic nerve a strip of waxed 

 paper, and keep it moist with normal saline. 



Carefully raise the nerve with the glass seeker and explore 

 it from end to end with minimal single induction shocks, the 

 effect of which have been tested first in the middle of the nerve, 

 and note if there be a difference of excitability at any point. 

 There usually is at one or two points find them. 



Be guided in your estimation of this by change in the mus- 

 cular effect evoked, such as increase, diminution, or absence of 

 contraction. 



Next cut the nerve at its spinal origin, and compare the 

 excitability at the cut end with that at a point near the muscle. 

 Repeat this from time to time. The cut end will presently 

 exhibit a greater excitability, which will fail later until it is 

 completely lost. 



A dying nerve at first rises and then falls in excitability, 

 finally losing it altogether. Remember that a nerve which is 

 drying becomes more irritable for this reason. 



Relative excitability of muscle and of nerve. Find the 

 minimal shock which will evoke a muscle twitch through the 



