THE NERVE-MUSCLE PREPARATION 25 



7. Mechanical excitation. The nerve (or muscle) can be stimulated 

 by mechanical means e.g., by tapping it gently or by allowing 

 mercury to drop (upon a nerve) from a height of three or four inches. 

 The effect of a mechanical stimulus is seen whenever a nerve is cut 

 or pinched, but any severe injury abolishes its conducting functions. 



8. Thermal excitation. The nerve (or muscle) is stimulated if it 

 be touched with a hot wire. 



9. Osmotic excitation. It can be stimulated by osmosis, as by 

 placing wet salt or pure glycerine upon it. The salt and glycerine 

 act by abstracting water. 



10. Excitation by drying. Drying acts as a stimulus, especially 

 to the nerve. When a 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 contraction ; this is, however, followed by paralysis. These 

 effects are at least in part due to the abstraction of lime salts from the muscle. 



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

 sign that be has not been careful to keep the nerve moist with Ringer's fluid. 



