CHEMICAL MODES OF STIMULATION. 465 



the sole factor which determines the exciting value of the salt, but that 

 this must be largely dependent upon the specific action of the chemical 

 compound upon the nerve. This is confirmed by the results of such 

 substances as KI, KBr, and KC1, which excite. motor nerves hardly 

 at all. 



The exciting value of equimolecular solutions of various neutral 

 salts, when applied to motor nerves, may be contrasted with one another 

 as follows : 



NaF>NaI> NaBr> NaCl 

 CcesCl>KubCl>KCl 

 BaCl>SrCl>CaCl 



The stimulation of afferent nerves by such salts presents some 

 remarkable features. In the first place, such stimulation, as evidenced by 

 reflex responses, has been found to be so capricious and uncertain as to 

 lead many investigators to state that sensory nerves are more or less 

 refractory to such excitation. Grutzner ingeniously utilised the well- 

 known smarting sensation which follows the application of saline solu- 

 tions to cuts upon the fingers. The production of this sensation is 

 undoubted evidence that the sensory nerves are stimulated ; on compar- 

 ing various salts in equimolecular solution, he obtained with sodium 

 salts results of which the following is a typical example : 



With Nal (14*9 per cent.) the sensation was evoked in five seconds, with 

 NaBr (10*2 per cent.) in ten seconds, with NaCl (5'8 per cent.) in fifty 

 seconds. This order is the same as that above referred to as prevailing with 

 motor nerves. But he found, further, that whereas potassium salts have 

 little exciting value for motor nerves, they are extraordinarily effective for 

 sensory nerves. Solutions of all the potassium salts produced the smart- 

 ing sensation when tested by the skin-cut method, and this when in a 

 state of concentration which, with equimolecular solutions of sodium 

 salts, was far too weak to be effectual. Similar results are obtained by 

 experiments on the conjunctiva, whilst, in the frog, salts of potassium 

 evoke reflexes, when applied to the skin, much more readily than 

 those of sodium. Moreover, the form of potassium salt which is most 

 effective is the chloride, while the chloride of sodium is the least 

 effective. 



The following table shows the comparative exciting efficiency of 

 various neutral salts when applied to afferent nerves : 



KC1 >KBr >KI 

 >NaBr >NaCl 



CaCl 2 >RubCl>KCl 



The extraordinary contrast between the marked exciting value of 

 KC1 when applied to afferent nerves, and its inadequacy as a stimulating 

 agent to motor nerve, is shown further by the following experiment. 

 The sciatic plexus of a narcotised frog is exposed and divided on both 

 sides. On applying potassium chloride to the central end of one plexus 

 and to the peripheral end of the other, reflex movements of the flanks, 

 etc., are evoked through the former, but no movement of the lower limb 

 is produced by the latter application. On the other hand, a similar ex- 

 periment with sodium chloride causes movements of the muscles whose 

 nerves are directly excited, followed after some little interval by reflex 

 effects in the flanks and other muscles. 

 VOL. n. 30 



