iv GENERAL PHYSIOLOGY OF NEEVOUS SYSTEM 219 



man that a few seconds after plunging the elbow into water at 

 the temperature of melting ice, a painful sensation is produced 

 over the whole cutaneous area served by the ulnar nerve, and later 

 a sensation of insensibility, which is undoubtedly due to diminished 

 conductivity in the cooled portion of the nerve. 



(&) Many soluble chemical substances act as stimuli when 

 applied to an exposed nerve. But the excitatory effects which 

 they induce are irregular in character, and in all probability 

 their action depends either on the removal of water from the 

 nerve, or on the specific action which they exert upon its 

 molecular state; or again upon the alteration or death of the 

 nerve at the points of contact. 



When a motor nerve is left to dry, its excitability rises at 

 first ; this is followed by a state of excitation expressed in a suc- 

 cession of small muscular twitches, or irregular tetanus ; lastly, 

 there is loss of excitability and conductivity. Up to a certain 

 point these effects are stronger in proportion to the length of 

 nerve exposed to desiccation. They vary also in different nerves, 

 and in different parts of the same nerve. If instead of dehydrat- 

 ing the nerve it is bathed in distilled water, the opposite phenomena 

 occur ; there is depression amounting to total loss of excitability. 



It is certain that some organic substances act as stimuli 

 when applied to nerve, by abstracting water from it. Such, e.g., 

 are glycerol, urea, the sugars, which stimulate motor nerves more 

 vigorously in proportion as they are more concentrated. As 

 regards the action of urea, Buchner noted that its prolonged 

 application is not, as is the case with other chemically exciting 

 substances, followed by loss of vitality in the nerve. 



Nearly all the neutral salts, if applied for some minutes to a 

 nerve, act as stimuli with an intensity approximately proportional 

 to their concentration ; too strong a solution rapidly inhibits or 

 destroys the excitability of the nerve (Griitzner). 



In order to obtain salt solutions perfectly comparable in their 

 effects, Griitzner employed equimolecular and not equivalent 

 solutions, i.e. solutions containing the same percentage doses of 

 salts. For the different sodium salts the scale of excitatory action 

 is NaF, Nal, NaBr, Nad. The molecular weights of these salts 

 are in an ascending order : NaF, 41'9 ; NaCl, 58'3 ; NaBr, 102'7 ; 

 Nal, 1494; and the percentage content of the equimolecular 

 solutions is NaF, 4-2; NaCl, 5-8; NaBr, 10'2; Nal, 14-9. From this 

 we may conclude that abstraction of water is not the sole factor 

 that determines the excitatory action of a salt, but that this 

 further depends upon the specific action of the chemical com- 

 pound upon the nerve. Griitzner demonstrated the same for the 

 salts of potassium, caesium, rubidium, barium, strontium, and 

 calcium. 



Grlitzner's experiments on afferent nerves with these salts are 



