THE MUSCLE-NERVE PREPARATION 



also diminishes the rate of conduction of nerve impulses along the 

 cooled portion (see p. 49). 



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 (see p. 20). It is best for this experiment 

 to place the secondary coil close to the primary and 

 to make use of the automatic interrupter. 



Excitation by condenser discharge. The dis- 

 charge of a condenser through a nerve, acts as 

 a stimulus. Arrange the apparatus as shown in 

 Fig. 31, in which c is a condenser made by covering 

 a sheet of glass with tinfoil on both sides. The 

 sheets of tinfoil are connected with a battery of two 

 or three cells for a few seconds, and then, by 

 turning the switch, are connected with the nerve, 

 the battery being cut off by the same movement. 



Mechanical excitation. A nerve (or muscle) can 

 be stimulated by mechanical means e.g., by tapping 

 it gently or by allowing mercury to drop upon it. 

 The effect of a mechanical stimulus is also seen when- 

 ever a nerve is cut or pinched, but a severe injury 

 abolishes its conducting functions. 



Thermal excitation. A nerve (or muscle) is 

 stimulated if touched with a hot wire ; or with a 

 wire cooled to below C. 



Osmotic excitation. A nerve can be stimulated 

 by withdrawal of water, caused by placing brine 

 or glycerine upon it. The salt and glycerine act 

 by abstracting water. 



Excitation by drying. The with- 

 drawal of water by drying also 

 acts as a stimulus to nerve. When 

 the nerve begins to dry, its muscle 

 twitches. 1 



Addition of water may also act 

 as a stimulus, especially with muscle. 

 If distilled water be injected into 

 the blood-vessels of an animal, all the muscles are thrown into contrac- 

 tion : this is, however, followed by paralysis. The effect is in part due 

 to the abstraction of calcium salts from the muscle. 



1 This twitching is a frequent source of puzzle to the beginner ; it is always a 

 sign that he has not been careful to keep the ner\ r e moist. 



FIG. 32. Diagram of mercury 

 dropper to produce mechani- 

 cal stimulation of a nerve. 

 Natural size. 



