464 NERVE. 



Chemical modes of stimulation. Many chemical substances 

 when applied locally to nerve act as stimuli ; the exciting effects are 

 irregular in character, and appear to be due partly to the withdrawal 

 of water from the nerve, and partly to the specific effect of the chemical 

 substance upon the molecular state of the nerve. 



Although the subject will be referred to again in the section which 

 deals with the influence of chemical substances upon nerve excitability 

 and conductivity, it is advisable at this stage to enumerate those forms 

 which appear to be most effective, and the mode of their employ- 

 ment. 



(1) The withdrawal of water by drying. The effect of drying a 

 motor nerve is to cause first increased susceptibility to excitation by 

 other modes of stimulation, then an irregular excitation which shows 

 itself in the attached muscles by flickering irregular muscular tetanus, 

 and finally loss of both excitability and conductivity. The effect 

 increases with the length of the nerve thus dried, and when the drying 

 is very rapid the final stage is reached so quickly that no responses 

 are obtained. 1 Different nerves and different portions of the same nerve 

 vary considerably as regards their susceptibility to stimulation by this 

 agency. Thus in the case of the sciatic nerve of the frog, the lower end 

 is far more readily excited than the upper, probably owing to the larger 

 amount of connective tissue surrounding the upper fibres, which pro- 

 tects the essential structure. A large number of chemical agents act by 

 withdrawing water, glycerine and sodium chloride being familiar 

 instances. 



(2) Neutral salts. Almost all the neutraFsalts cause nerve excitation, 

 whether they are applied by allowing a nerve to lie for some minutes in 

 a bath containing an appropriate solution of the salt, or whether the salt 

 is applied locally to a portion of the nerve. The exciting value varies 

 directly with the degree of concentration, until a limit is reached at 

 which the nerve functions are suspended, if not destroyed, by the rapid 

 action of the strong solution. The investigation has been largely carried 

 out on motor nerves, particularly the sciatic nerve of the frog, the 

 muscular response being taken as the index of effective excitation, 

 which resembles in its general characters that produced by drying. 



Sodium chloride is the form which has been most frequently 

 employed, and the salt tetanus evoked by its application has been 

 studied by Eckhard, Kolliker, and others. An extended inquiry into 

 the exciting value of different salts has been made by Griitzner. 2 In 

 order to obtain solutions of different salts which should admit of accurate 

 comparison, the solutions used were not of equal percentage, but equimole- 

 cular. Thus, in comparing NaF, NaCl, NaBr and Nal (the vary- 

 ing molecular weights of which are respectively 41*9, 58*3, 102'7, 149 4), 

 equimolecular solutions are those containing corresponding proportions 

 by weight of the salts, namely, 4'2 NaF, 5 '8 NaCl, 10 '2 NaBr, 

 14'9 Nal. The effectiveness of these solutions for stimulating the 

 sciatic nerve of the frog was found to be in the following order : NaF, 

 Nal, NaBr, NaCl. The action of NaF is very intense; if this is 

 excepted, the remaining salts act more strongly the higher their mole- 

 cular weight, and even with equal percentages the same relationship 

 holds good. It therefore appears that the withdrawal of water is not 



1 Haiiess, Ztschr.f. rat. Med., 1859, Bd. vii. 



2 Griitzner, Arch.f. d. yes. PhysioL, Bonn, 1893, Bd. liii. S. 83. 



