222 PRACTICAL PHYSIOLOGY 



CHAPTER LXIII. 

 MULLER'S LAW OF THE SPECIFIC ENERGY OF NERVES. 



The Law of the Specific Energy of Nerves propounded by Johannes 

 Miiller states that each sensory nerve gives rise to its own particular 

 sensation, whatever may be the means whereby it is excited. Thus the 

 retina only gives a sensation of sight, whether it be stimulated by 

 light, a blow or an electrical shock. 



This law can be demonstrated by the following experiments. 



Sight. (i) Two clinical electrodes moistened with strong saline 

 solution are connected by means of a key with a Daniell cell; one 

 electrode is placed upon the forehead, the other upon the nape of the 

 neck. On make or break of the constant current the subject will have 

 a sensation of a flash of light. 



(ii) The retina can be stimulated mechanically by pressure .on the 

 sclerotic. A sensation of light will be experienced. 



Taste. The end-organs of taste can be stimulated not only by sapid 

 substances, but also by mechanical and electrical means, (i) Gentle 

 tapping of the front of the tongue gives a sensation of a sweet taste. 



(ii) When the free ends of two wires connected with a Daniell cell 

 are placed upon the tongue and the current is opened or closed, a 

 sensation of taste is experienced. This experiment can be performed 

 with suitable unpolarisable electrodes, so that the objection, that 

 electrolysis is produced and the resultant ions are tasted, may be con- 

 sidered negatived. Moreover, weak faradising shocks, which would 

 cause but little electrolytic action, also give rise to sensations of taste. 



The anode appears to produce an acid taste, the cathode an alkaline 

 taste. 



Smell. The olfactory nerve-endings give rise to a sensation of 

 smell when they are stimulated with an electric current. The experi- 

 ment can be performed in the following way. The electric current is 

 sent through the nose by one electrode connected with the nose by 

 filling the nasal cavity with normal saline solution ; the other electrode 

 is placed on the forehead. The odour is said to resemble that of 

 phosphorus. 



Cutaneous Sensations. Sensations of touch, cold, warmth, and 

 pain can be evoked by gentle application of the point of a metal rod to 

 the skin of the hand. The areas or spots which on stimulation give 

 rise to the different sensations should be mapped out with ink. 



