LI.] EXCITABILITY OF NERVE. 255 



(a.) Arrange the apparatus as in fig. 178. Dissect out the whole 

 length of the sciatic nerve with the leg attached. Lay the nerve 

 on two pairs of electrodes, A and B, one near the muscle arid the 

 other away from it, and as far apart as possible. Two pairs of 

 wires thrust through a cork will do quite well. 



(6.) Stimulate at A with a current that gives a minimal contrac- 

 tion. Reverse the commutator. Stimulate at B, a stronger 

 contraction is obtained, because the excitability of a nerve is 

 greater farther from a muscle or nearer the centre. Instead of 

 using single shocks, repeated shocks by means of Neef's hammer 

 may be used. 



FIG. 178. Scheme for the Unequal Excitability of a Nerve. 



2. Effect of Temperature on Excitability of a Nerve. 



(a.) Fix a nerve-muscle preparation on a crank-myograph, so as 

 to record on a revolving cylinder provided with an automatic break- 

 key placed in the primary circuit of an induction coil, and so 

 arranged as to give only feeble break shocks. 



(b.) Bring a test-tube filled with water at 80-90 C. near the 

 nerve, where the electrodes lie on it. Soon the contraction 

 increases and may become maximal. 



(c.) Eemove the source of heat and the contractions become less, 

 i.e., the excitability falls. 



((/.) Similar results may be obtained by the direct application 

 of warm normal saline to a nerve. 



(For other kinds of nerve fibres see " Effects of stimulation and 

 of changes in temperature upon irritability and conductivity of 

 nerve fibres," by Howell and others, Journal of Physiology, xvi. 

 p. 298.) 



3. Salt Increases the Excitability of a Nerve. 



(a.) Arrange a nerve-muscle preparation as in 2, and determine 



