1153 



When tlie rlieobaëis and the chronaxia are known we find from them 



1 

 Hoorweg's initial constant a, or its reciprocal value as quantUy 



a 



coiistunt. 



It is generally much less important to know the rheobasis than 

 the chronaxia. The latter seems to be a trne constant, whereas the 

 rheobasis depends on the surface of the electrodes, the place where 

 the^' ai'e applied, the condition of the skin, arid even on the duration 

 of the constant current, used in measuring- it. With the same 

 electrodes 1 found it in one case to be 4.4 milliampere with the 

 condensor method, 6.1 milliampere with the induction coil-method, 

 against 2.6 milliampere as measured with the constant current, the 

 three measurements being made immediately one aftei' the other. 

 But in all three cases the chronaxia came out as 0.00008 second. 

 It seems possible that also differences in the resistance of the body 

 might be responsible for the differences in the rheobasis. 



The rheobasis is easily determined and is even a matter of routine 

 work in neurological praxis. But little attention if any is given to 

 the chronaxia. Only if the methods ai-e simplified there may be 

 some chance of a more extensive use of this way of stating the 

 excitability of muscular tissue and motor nerves. Below I give two 

 simplified methods for directly measuring the chronaxia, either 

 by two condensor-discharges or by two measurements with an 

 ordinary not-graduated induction coil. In every case the chronaxia 

 is found without any calculation or with merely one division. 



Condensor-inethod. 



Muscles and nerves i-espond in the same waj' to discharges of 

 condensers of different capacity C if the Voltage V in each indi- 

 vidual case be 



^ 1 



F=-ie4— ........ (2) 



(I at 



in which R is the resistance of the circuit, ^i and <( the above- 

 mentioned constants. This relation was found by Hookwkg. It may 

 be calculated from (1) by putting 



which is the well-known formula for a condensor discharge through 

 a non-conductive resistance. If the expression is integrated and we 

 take for y a unity-effect = 1 we get formula (2j. 



We first make a measurement with a capacity C\ and find a 

 voltage r which first gives a minimal contraction. Next we take 



