208 



Dr. A. D. Waller. 



excitation is kathodic. Dubois (4, 5), of Bern, studied the law of con- 

 tractions upon human nerve, and came to the conclusion that the 

 greater and smaller effects of condenser discharges are governed by the 

 greater and smaller quantities of electricity in play. Hoorweg (11, 12), 

 Cybulsky and Zanietowsky (9, 10) studied the effects more closely 

 upon frog's nerve ; the first-named observer concluded that the effects 

 are a function of intensity ; while the last two observers, from very 

 similar observations, concluded that the magnitude of excitation is a 

 function of energy. Salomonsen (8), Boudet (2), and D'Arsonval (6, 7) 

 come to the same conclusion, and the last-named observer makes use of 

 an expression — " la characteristique de 1'excitation " — very similar to 

 the designation that I had been independently led to adopt, in ignor- 

 ance of its previous use in a different sense. 



The problem is, I think, to be considered from the following a priori 

 point of departure : — A stimulus arouses excitable matter by reason of 

 its actual energy and not of its mere quantity. A weight per se is not 

 a stimulus, but a weight dropped from a height acts as a stimulus that 

 is to be expressed in terms of energy. One gram fallen through 1 cm. 

 strikes and stimulates with an actual energy of 1000 (or more precisely 

 981) ergs. Similarly, as regards the electrical stimuli afforded by the 

 (charge or) discharge of a condenser, it is neither the quantity (cou- 

 lomb) nor the pressure (volt) that alone gives measure of the stimulus, 

 but the energy (Joule or 10 7 ergs) of that quantity at that pressure. 

 A nerve (or other excitable tissue) may be struck and stimulated by 

 an electrical energy of so many ergs. And we are adequately 

 acquainted with the physiological value of a stimulus when we 

 know : — First, its absolute value in ergs or fraction of an erg. Second, 

 the rate at which such energy impinges upon and sets in motion the 

 excitable molecules under investigation. 



The absolute value of a true minimal stimulus in ergs or fraction of 

 an erg, is ascertained by determining the optimum capacity and vol- 

 tage at which a minimal response is obtained. The energy E in ergs 

 = 5 FV 2 , where F is in microfarads and V in volts. 

 Its rate of impact depends upon the rate of (charge or) discharge 



= x a constant, where E, denotes the resistance in circuit in 

 FIv 



ohms, and can therefore be expressed by a number that will be higher 

 or lower according as the rate of discharge is greater or smaller.* 



* With the same units as above, and with the unit of time = 10 — 6 second, the 

 constant is 0'8687. In the subjoined data it has been taken as 868700 for the unit 

 of time = 1 second. 



For certain ends it is convenient to indicate the rate of discharge of energy by 

 stating the time necessary for its fall to any given fraction \\n of its original value, 



according to the formula t — KE. ^ n - ; e.g., with a capacitv of O'Ol microfarad, 



V. Iriry f 



