24 



EXPERIMENTAL PHYSIOLOGY 



distance from the primary, and make a note as to the respective 

 positions of the secondary coil. This gives a measure of the 

 excitability of the nerve. Its excitability to tetanisation (use the 

 Neef's hammer and the Helmholtz wire) is determined in the same 

 way. 







3. Reaction of nerve and muscle to galvanic and faradic excitation. It the 

 above experiment be tried with a nerve-muscle preparation and with nerve-less 

 muscle (the nerves within the muscle are paralysed by curari), it will be found 

 that the nerve will react to a weaker stimulus than the muscle when induction 

 shocks are employed ; but if the make and break of a galvanic circuit be used 

 as the stimulus, the muscle will respond to the weaker stimulus. Plain muscle 

 shows this even more strikingly. 



4. Gotch's experiment. Cooling a nerve causes it to react to galvanic and 

 faradic stimulation in the same way as muscle. The upper end of the nerve of 

 a nerve-muscle preparation is cooled by laying it on a tube through which ice- 

 cold water is passing ; the muscle itself must not be allowed to come anywhere 

 near the cold tube. If, now, the cooled part of the nerve be stimulated, it will 

 be found to be less sensitive to faradic and more sensitive to galvanic stimu- 

 lation than a part nearer the muscle which has not been cooled. Cooling the 

 nerve also diminishes the rate of conduction of nerve impulses along the cooled 

 portion (see p. 48). 



5. Unipolar induction. It is possible to stimulate the nerve when 

 it is connected by only one wire with the secondary coil ; hence the 

 necessity for using a short-circuit key to prevent unipolar induction 

 (seep. 19). It is best for this experiment to place the secondary coil 

 close to the primary and to make use of the automatic interrupter. 



6. Excitation by condenser discharge. The discharge of a condenser through a 

 nerve acts as a stimulus. Arrange the apparatus as shown in fig. 31, in which 



PIG. 31. STIMULATION OP NERVE BY CONDENSER DISCHARGE. 



C is a condenser made by covering a sheet of glass with tinfoil on both sides. 

 The sheets of tinfoil are first connected with the battery for a few seconds, and 

 then, by turning the switch, are connected with the nerve, the battery being 

 cut off by the same movement. 



