220 PHYSIOLOGY CHAP. 



interesting. In view of the uncertainty of the results when 

 reflexes were taken as the index of excitability it occurred to 

 him to utilise the burning sensation felt on applying equimolecular 

 salt solutions to a cut on the finger. With sodium salts his results 

 were as follows: with Nal (14'9 per cent) sensation is aroused 

 after 5 sees.; with NaBr (10'2 per cent), after 10 sees.; with NaCl 

 (5-8 per cent), after 15 sees. Sensory nerves are accordingly stimu- 

 lated in the same order as motor nerves. But on using potassium 

 salts Griitzner observed an interesting difference in the reaction of 

 motor and sensory nerves. These salts have only a slight stimulat- 

 ing effect on motor nerves, but act very powerfully on sensory 

 nerves ; for the latter potassium chloride is the most active, 

 sodium chloride is the least active of all. This important point 

 can be demonstrated by the following experiment. If the sciatic 

 plexus of an anaesthetised frog is divided on both sides, and 

 a solution of KC1 applied to the central end of one plexus and 

 the peripheral end of the other, reflex movements are seen in the 

 anterior limbs and trunk, while no contractions appear in the 

 muscles of the excited posterior limb; on repeating the experi- 

 ment with NaCl, movements are seen in the muscles of the 

 directly excited limb, while movements of the reflexly excited 

 muscles only appear after an interval. This difference can be 

 interpreted to mean that KC1 is. better able to excite in the 

 afferent direction, i.e. to awaken the activity of the central organs, 

 while NaCl is more able to excite along efferent paths, i.e. to stir 

 up the activity of the peripheral end-plates. Moriggia, on the 

 contrary, found that NaCl (0'4 per cent) excited the sensory and 

 not the motor fibres. 



The results of experiments on the excitatory action of the 

 basic compounds do not agree. Eckhard and Klihne observed that 

 even very weak solutions (O'l per cent) of NaOH and KOH were 

 exciting to motor nerves ; Griitzner, on the contrary, found that 

 their stimulating action was very weak, while larger doses had a 

 destructive effect. Ammonia kills the nerve without exciting it. 



Inorganic acid compounds in general have a stimulating action 

 proportional to their chemical avidity. Griitzner found that 

 nitric and hydrochloric acid stimulated in weaker solutions than 

 sulphuric acid. The organic acids excite only in concentrated 

 solutions, and some of them (e.g. oxalic acid) destroy the vitality 

 of nerve without exciting it. 



The salts of the heavy metals again affect the vitality of 

 nerve, without any previous stage of excitation, but according to 

 Eckhard and Klihne, zinc chloride, zinc sulphate, and ferric 

 chloride are exceptions to this rule. 



(e) Every one knows that mechanical factors, e.g. compression, 

 shock, crushing, pulling, cutting, puncture, produce excitation 

 when they act on nerve at a certain rate and with a certain 



