ADVANCED EXPERIMENTAL PHYSIOLOGY 359 



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. 

 Mechanical stimulation with a metal rod warmed to the same tempera- 

 ture as that of the skin of the hand will give rise to sensations of 

 touch, temperature, or pain according to the area stimulated. Miiller's 

 law is thus demonstrated in the case of these sensations. 



There is some doubt whether there are specific nerves -for painful 

 sensations ; it may be that excessive stimulation of any sensory nerve 

 causes pain. 



The sensibility of different parts of the skin to tactile sensations 

 may be investigated by gently touching the surface with the blunted 

 points of a pair of compasses. The distance between the points is 

 gradually increased, and the blindfolded subject is told to say whether 

 the sensations appear to be those arising from one or two points of 

 contact. The distances between the points of the compasses are 

 measured upon the millimetre scale of an induction coil. 



The tips of the fingers will be found to be much more sensitive than 

 the back of the hand, and the latter more sensitive than the forearm. 



CHAPTER XXXII. 



THE RATE OF DISCHARGE OF NERVOUS IMPULSES FROM THE 

 CENTRAL NERVOUS SYSTEM. 



THE rate at which nervous impulses can be discharged by the central 

 nervous system can be investigated in the frog by exciting the nerve 

 cells by means of a drug such as strychnine and recording the resulting 

 incomplete tetanus ; or in man by the record of the contraction of a 



