580 FUNCTIONS OF THE CEREBRO -SPIN A L NERVOUS SYSTEM. 



to the contents. The sensory nerves appear to be much more readily acted 

 on, and by a much greater range of chemical agents, than the motor nerves ; 

 powerful stimuli for the latter are, however, found in solutions of Soda and 

 Potash, even when not exceeding 2 per cent., in those of Nitrate of Potash 

 and of Hydrochloric acid, containing 20 per cent., and in concentrated 

 solutions of Ammonia and Alcohol (90 per cent.) ; the latter, however, 

 rapidly killing the nerve at the point where they have been applied. Bi- 

 sulphide of carbon, the ethereal oils, and concentrated mineral acids kill the 

 nerve at once, without producing convulsions. 1 Water applied to nerve- 

 trunks excites no muscular contractions, but if injected into the vessels dis- 

 tributed to a muscle induces very violent convulsions, which are believed by 

 Sch iff to be due to the direct effect of the water upon the delicate termina- 

 tions of the nerves in the muscles. Withdrawal of water from the nerves, 

 if effected gradually, produces no effect, but if accomplished rapidly, pro- 

 duces tetanus. 2 As regards thermic irritation as a stimulus for the nerves, it 

 has been shown by Weber that the sensory nerves do not perceive positive 

 temperature, but only variations in the degree of heat of external objects. 

 The influence of variations of temperature on motor nerves has been investi- 

 gated by Eckhard, who has shown that a temperature of about 130 3 F. pro- 

 duces convulsions in frogs, which, however, soon cease; the irritability of 

 the nerves being then lost, though upon cooling they occasionally regain it. 

 Sudden exposure to a temperature of about 25 F. also produces convulsions. 3 

 Temperatures intermediate to these limits do not occasion convulsions, but 

 the nerve soon loses its irritability when exposed to those near either the 

 higher or the lower extreme. 



470. During life, and in the healthy state, the excitability of a motor 

 nerve gradually diminishes from its origin to its distribution in the muscle, 

 so that a stimulus of determinate intensity produces a more energetic con- 

 traction of a muscle in proportion to the distance from the muscle that it is 

 applied to the nerve, or, as it is expressed by Pfluger, in proportion to the 

 length of the myopolar portion of nerve. Thus Budge found that to pro- 

 duce the same effect, i. e., the same amount of contraction it was necessary 

 to apply a stimulus of more than double the strength close to a muscle, than 

 was requisite if the stimulus were applied to the portion of nerve near its 

 origin. The question arises whether the nerve is itself more excitable at the 

 parts nearer the centre, or whether the stimulus does not gain force in its 

 descent, and produce, like an avalanche, an effect greater in proportion to 

 the distance it has traversed. Pfluger expresses himself decisively in favor 

 of the latter supposition. Yet it seems opposed to our ordinary notions of 

 the resistance offered to the passage of currents by conductors, and the phe- 

 nomenon appears to be more readily explicable on the supposition that the 

 structure of the nerve, and especially of its sheath, may be more delicate 

 in those parts which are more protected, and that it consequently there 

 more readily responds to impressions. After death the excitability of the 

 nerves dies out centrifugally, the part in proximity to the muscles remaining 

 longest excitable ; but Rosenthal has .shown that immediately after death 

 the excitability of the nerve is for a short period considerably exalted, and 

 that this exaltation is shorter in duration in proportion to the distance of 

 the portion of nerve examined from the muscle. According to Oehl, 4 if the 



1 SehiflT, Physiologic, 18'>9, p. 101. 



2 Mai-less, Zeitschrift f. Rat, Mcd., Bel. vii, p. 219. 



3 EcUhard, Zcitschrit't 1'. Hut. Mod., 13d. x, p. 105; and Harless, ibid., Bd. viii, p. 

 122. 



4 Annali Universal!, torn, cxc, 1864, p. 465; 1865, p. 601, and Lombard, Med.- 

 Chir. Kuv., Jan. 18(i'J. 





