466 NER VE. 



These differences in the apparent excitability of afferent and efferent 

 nerves to chemical stimulation may be dependent upon the difference 

 between the responding end-organ present in the two cases. On this 

 hypothesis, even though all the nerve fibres are excited, the indication 

 of such excitation in the case of a motor nerve, i.e. the resultant 

 muscular response, is one which is readily evoked by the transmitted 

 excitatory states produced by sodium salts, but not by those started by 

 potassium salts ; on the other hand, in the case of afferent nerves, the 

 excitatory state aroused by the potassium salt is of a character to evoke 

 central (reflex) activity, and this in a more marked degree than is the 

 case with that due to sodium salts. Any explanation must include 

 those phenomena of isolated nerve, in which an electrical reponse is 

 the index of activity. These are referred to in the section dealing 

 with the electromotive phenomena of nerve. 



(3) Stimulation by bases. — Considerable discrepancy exists as to the 

 stimulating effects upon motor nerves of such alkalies as NaHO, KHO, 

 and ammonia. According to the majority of observers, the first two 

 will excite a motor nerve, even when weak, e.g. 0"1 per cent. 1 (On 

 the other hand, Griitzner found that KHO rarely if ever excites 

 the motor nerve.) As regards ammonia, the great majority of 

 observers are agreed that it does not excite such nerves. In all cases, 

 these alkalies, if at all concentrated, rapidly destroy all functional 

 activity, — a circumstance which may account for the discrepancies in 

 the observations. 



As regards afferent nerves, Griitzner found that all three alkalies ex- 

 cited when tested by the skin-cut method, and that their comparative 

 exciting efficiency was as follows, Ammonia, KHO, NaHO, thus showing 

 again the marked contrast between the central system response and 

 that of the peripheral muscle to chemical excitation of the centripetal 

 and centrifugal nerves respectively. 



(4) Stimulation by acids. — Acids excite both motor and sensory 

 nerves. 1 As regards inorganic acids, their comparative exciting value 

 is in strict accordance with their chemical avidity. Thus HNO ;j and 

 HC1 excite in weaker solution than H 2 S0 4 , whilst H 3 P0 4 excites very 

 feebly. 2 The organic acids excite only when concentrated, and some 

 of them, such as oxalic acid, impair vitality without excitation. 



(5) Stimulation by salts of the heavy metals. — The majority of 

 metallic salts destroy nerve vitality without excitation. Mercuric 

 nitrate, zinc chloride, zinc sulphate, and chloride of iron are exceptions. 

 These excite, when solutions are used of a concentration which causes a 

 comparatively slow injury. The first is by far the most potent, and 

 the others differ as regards exciting efficiency in the order above 

 given. 1 



(6) Stimulation by organic substances. — Glycerin, if concentrated, 

 excites motor nerves, probably through the withdrawal of water ; and, 

 as in the case of drying, the lower end of the frog's sciatic is far more 

 readily stimulated than the upper end. Urea and sugar, if sufficiently 

 concentrated, excite in a similar w T ay ; the former is remarkable inas- 

 much as it does not produce the eventual loss of vitality which is the 

 common result of the prolonged action of other exciting chemical sub- 



1 Eckhard, Ztschr, f. rat. Med., 1851 (2), Bd. i. ; Kiihne, Arch./. Anat., Physiol, n. 

 wissensch. Med., 1859. 

 - Griitzner, loc. cit. 



