GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 155 



and in the same nerve under different conditions. In the medullated nerves 

 of the frog it lasts 0.008 to 0.024 second ; l in the non-medullated nerves of 

 the pike the rise of negativity requires 0.08-0.029 second and the fall 0.40- 

 1.2 second. 2 The rate of conduction is slowed by cold, and this at the same 

 time lengthens the accompanying electrical change. This fact has been 

 made use of to ascertain that in the uninjured nerve, as in the muscle, there 

 is a diphasic current of action spreading in both directions from the point of 

 excitation. 



The strength of the electrical change which takes place in a nerve when 

 it is excited is the best evidence which we have of the activity of the nerve. 

 The physiological structures which the nerve normally excites obey laws 

 peculiar to themselves, and are liable to give results which are open to mis- 

 interpretation. For example, if a nerve be stimulated in the middle, the 

 condition of activity aroused spreads in both directions, and causes at the 

 one end a contraction of the muscle, and at the other a negative variation of 

 the current of rest, which may be observed with a galvanometer. If the 

 nerve be excited many times in succession, the height of the muscular con- 

 tractions is seen to decrease while the electrical changes show no sign of 

 fatigue. The decrease in the height of the contractions is really due to the 

 fatigue of the nerve ends and the muscle, and the constancy of the electrical 

 changes is the truer expression of the state of the nerve. 



A difficulty presents itself here, however : the negative variation currents 

 observed in such an experiment may be so very regular as to suggest that 

 they are phvsical rather than physiological phenomena. That they are not 

 purely physical can be ascertained by subjecting the nerve to influences of a 

 type to alter the physiological activity of the protoplasm, without perma- 

 nently, or indeed markedly, altering its chemical and physical structure. A 

 study of the effect of anaesthetics on the nerve is especially instructive. In 

 general, anaesthetics are found iirst to heighten, later to lower, and finally to 

 destroy the irritability. 



If the anaesthesia is not carried too far, the nerve may completely recover 

 its function on the removal of the drug. Waller 3 describes the following 

 experiment : A fresh nerve is placed on two pairs of* non-polarizable electrodes 

 in a moist chamber. One pair is connected with the galvanometer which is 

 to record the current of action, the other pair is connected with an induction 

 apparatus, and brings the exciting current to the nerve. Induction shocks 

 of equal strength are sent into the nerve at regular intervals, and the extent 

 of the currents of action awakened is noted. Alter the electrical response 

 of the nerve has been tested, fumes of ether (diethyl oxide) are blown through 

 the chamber. At first the electrical response is found to be increased, later 

 it decreases, and at the end of three or four minutes it is altogether lost. If 

 air be now allowed to enter the chamber, the current of action reappears, 



1 Biedermann : Eleetrophysiology, translation by F. A. Welby, 1898, vol. ii. p. 260. 



2 Gartner: Pfliiger's Arehiv, 1899, Bd. lxxvii. S. 198. 



3 Waller: Lectures on Animal Electricity, 1897, Lecture 11. 



