1912.] The Process of Excitation in Nerve and Muscle. 497 



redistribution of electric potential in the nerve (the electric response) and 

 the failure of a second stimulus to produce like effects for a period of a few 

 thousandths of a second (the refractory state). On the arrival of the 

 propagated disturbance at the muscle all the phenomena which occurred in 

 the nerve are repeated there, and, in addition, the muscle undergoes a 

 mechanical contraction and heat is liberated. We have to enquire how 

 each of these phenomena is related to the propagated disturbance. 



A. The Local Excitatory Process. 



Consider, then, the first link in this chain, the setting up of the propagated 

 disturbance by the electric current. Is this all, or does the current itself 

 bring about some intermediate change which then provides the necessary 

 condition for starting the propagated disturbance ? The question is of 

 fundamental importance to our analysis, because if such a preliminary 

 process is involved we may hope, from a study of the conditions to which 

 an adequate stimulating current must conform, to learn its physico-chemical 

 nature ; we shall then be in possession of the conditions within a tissue by 

 which the propagated disturbance is started, and shall have made one step 

 towards the identification of that disturbance. 



The classical method of dealing with this problem has been to show that 

 various substances, such as carbon dioxide, ether, and alcohol, which 

 gradually suspend the functional activity of nerve, alter at different ' rates 

 the ease with which a propagated disturbance already started can travel 

 along the nerve, and the ease with which such a disturbance can be initiated 

 by an external stimulus. In the usual terms, there is a difference in the 

 rates of abolition of " conductivity " and " excitability." The method, used 

 first by Grunhagen,* and later by many observers who followed him, is to 

 treat with any of the narcotics only a part of the length of the motor nerve 

 innervating a muscle, leaving the portion more remote from the muscle 

 unaffected. From time to time during the progressive action of the narcotic 

 determinations are made of the strength of stimulus required to cause a 

 •contraction of the muscle both when the stimulus falls actually within the 

 narcotised region of the nerve, and when it falls on the normal part beyond. 

 If the narcotic merely renders the nerve less easily excitable, the stimulus 

 applied within the narcotised region will need to be strengthened, whereas 

 that applied to the normal region will need no alteration. If, on the other 

 hand, the narcotic lowers the ability of the nerve to conduct, then the 

 stimulus applied in the normal part may become ineffective, because the 

 propagated disturbance which it sets up has to traverse the narcotised 

 * Griinhageii, ' Arch. f. d. ges. Physiol.,' 1872, vol. 6, p. 180. 



