132 THE PHYSIOLOGY OF NERVE 



long before it actually causes an interruption of the fibrillae of the axis- 

 cylinders. 



The fact that temperature influences the speed of conduction has 

 already been established by the earlj^ experiments of Helmholtz. The 

 relationship between these two factors is a direct one, ^.6., the higher 

 the temperature, the more rapid the conduction, but this rule is appH- 

 cable only within physiological limits. In the case of the motor nerves 

 of man, variations between 30 and 90 m. per second have been obtained. 

 This is also true of the nerves of invertebrates, those innervating the 

 claws of the lobster, showing a velocity of 6 m. at 10° C, and of 12 m. 

 at 20° C. The motor fibers of the sciatic nerve of the frog cease to 

 conduct at 41-44° C, but may recover if the temperature is again 

 lowered. At 50° C. their conductivity is lost altogether. It is also 

 of some interest to note that the velocity of the nerve impulse follows 

 the van't Hoff law for chemical reactions, because, as has been shown 

 by Snyder, 1 a rise in temperature of 10° C. approximately doubles the 

 conduction. This fact may be employed as a proof that conduction 

 by nerve entails certain chemical changes, because most physical pro- 

 cesses present for this range of .temperature a relationship of only 1 :1 

 or a relationship barely above unity. 



Unusual changes in temperature, and especially those beyond 

 physiological limits, cannot be considered as constituting pure thermal 

 influences, because they are prone to injure the nerve tissue by bringing 

 about a loss of water or certain differences in its electrical tension. 

 In this category belongs the abolition of conduction in consequence of 

 cauterization and extreme cooling. Thus, the application of ice to 

 the region of the ulnar nerve at the elbow results at first in sensations 

 of pain and finally in a complete loss of sensations. 



A nerve may be kept in a physiological condition by frequently 

 moistening it with normal saline solution, but its complete immersion 

 in this solution (0.6 per cent.) is generally followed by phenomena 

 of excitation which, however, do not appear if Locke's or Ringer's 

 solution is employed instead. Overton^ has shown that nerve- 

 muscle preparations retain their functional qualities in the latter 

 even after 15 to 20 days. Immersion in water diminishes the irrita- 

 bility of nerve. Moreover, it is a matter of common observation that 

 its drying leads to violent contractions of the muscle which, to begin 

 with, are clonic in character but soon become tetanic. Acids do not 

 irritate unless concentrated;^ alkalies, on the other hand, stimulate 

 even in solutions of 0.8-1.0 per cent. According to Mathews,'' the 

 different solutions of the sodium salts act as exciting agents only in 

 high concentrations, but some of them also stimulate when isotonic 



^ Am. Jour, of Physiol., xxii, 1908, 179; also see: Ranitz, Pfliiger's Archiv, 

 Ixviii, 1907, 601. 



2 Pfluger's Archiv, cv, 1904, 256. 



3 Kiihne, Archiv fiir Anat. und Physiol, 1860, 315. 

 ^ Am. Jour, of Physiol., xi, 1904, 455. 



