THE TIME RELATIONS OF THE CHANGE. 



47i 



application. With any given intensity the stimulating effect of the 

 onset or cessation of a galvanic current is largely determined by the time 

 relations, i.e., by the rapidity with which either of these is effected. 

 This is readily demonstrated by a rheonome, an apparatus designed so as 

 to constitute a varying rheochord. 1 



Fleischl's rheonome consists of a narrow circular canal, filled with saturated 

 solution of zinc sulphate ; a battery is connected with two fixed points in this 

 canal at opposite ends of one diameter, whilst two revolving contacts dipping 

 into the canal are connected with the derived or exciting circuit, and thus with 

 the nerve (Fig. 253). At each complete revolution the difference of potential in 

 the derived circuit is augmented and decreased in alternate directions with a 

 rapidity dependent upon the rate of revolution, and since the apparatus 

 provides for equal revolutions in equal times the variation is linear. With 

 a very slow rate of revolution the current increase and decrease can be 

 effected without causing a nerve excitation which shall be adequate to evoke 



Fig. 253. — I, Diagram of rheonome. Circular trough containing saturated solution 

 of ZnS0 4 , connected by zincs at ZZ' with battery circuit ; the revolving 

 arm (A) dips by its end contacts into the trough ; when it reaches the posi- 

 tion (A') the two points of contact in the trough are equipotential. 77, 

 Diagrammatic representation of the variation in current, through the wires 

 attached to the ends of the revolving arm, when this is slowly rotated. 



a muscular contraction ; as the rate increases the stimulus becomes effectual, 

 evoking minimal and subsequently maximal contractions when a certain rapidity 

 of development has been attained. The conditions are such that the maximum 

 of current intensity is the same at whatever rate the revolving contacts 



move, hence the rapidity of development (j^,) is the effectual factor in 



determining the exciting efficiency. 



The difference between the make and the break induced currents 

 is largely due to varying rapidity of development ; the break induced 

 current, reaching its maximum more rapidly than the make, is more 

 effectual. 



The muscular nerves of the frog and of most mammals are differ- 

 entiated so as to he extraordinarily susceptible to rapid variations of 

 current intensity ; the excitation of such a nerve by the static spread 

 from the neighbourhood of the poles of a frictional machine or by 

 telephone currents are illustrations of such susceptibility. 



But, in spite of this, du Bois-Keymond's statement cannot be 

 accepted as a fundamental law of excitation for excitable tissues 



1 Fleischl, Sitzungsb. d. k. Alad. d. Wissensch., Wien, 1877, Bd. lxxvi. Abth. 3, S. 3 ; 

 v. Kries, Arch. f. Physiol., Leipzig, 1884. 



