358 PRACTICAL PHYSIOLOGY 



revolving drum ; the closure of the key K 2 by the subject 

 of the experiment brings the chronograph to rest. The number of 

 vibrations recorded upon the drum gives the reaction time for sound 

 in T J^ths of a second. 



The total reaction time in this experiment is composed of (1) the 

 time taken by the sound to reach the ear ; (2) the time taken for the 

 reception of the stimulus by the sensory endings of the auditory nerve 

 and the transmission of the nervous impulse to the sensory area ; (3) 

 the time for the transmission to the higher centres so that volitional 

 impulses may be started in the cerebral motor centres ; (4) the time for 

 the propagation of those motor impulses to the nerve cells of the spinal 

 cord ; (5) the time required for the generation of impulses in these cells 

 and their passage down the motor nerves to the muscles of the hand ; 

 and (6) the latency of the contraction of those muscles. 



The reaction time for sound is about 0*150 second, for light 0'195 

 second, and for touch about 0'14:5 second. 



CHAPTER XXXI. 

 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 (page 109). 



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 



