PHYSIOLOGY OF THE INVERTEBRATA. 333 



was attached to the dactylopodite o£ the chela (Pig. ()€)^ and 

 all firmly fixed, by the aid of bands, upon the horizontal plate 

 of the myograph. The dactylopodite was held by means of a 

 horizontal elastic spring ; the object being to keep it away 

 from the other portion of the claw. A pair of platinum 

 electrodes were applied upon each of the two portions of the 

 exposed nerves. The four wires from these electrodes were 

 fastened to the wires of the induction coil by means of a 

 commutator, which allowed the changing of the electric shock 

 into one or other of the pairs of electrodes, and of exciting 

 the nerve in its nearest or furthest point from the muscle. 

 Fig. 66 shows the arrangement of the apparatus used in these 

 experiments. 



After ascertaining that the muscle reacts suflBciently to the 

 excitation of the nerve, and that the point of the style marks 

 properly on the smoked paper of the registration cylinder, 

 Fredericq and Vandevelde arranged the commutator in such 

 a manner that the induction shock could not act upon the 

 nerve, and then allowed the cylinder to turn until it attained 

 its normal velocity. The point of the style traces upon the 

 paper a horizontal line, an absciss of which the turns are 

 reproduced exactly. The cylinder continuing its revolutions, 

 the commutator was so arranged as to excite the point h of 

 the nerve at the moment when the two points of the needles 

 which close the circuit touch each other. The muscle con- 

 tracted, and the style gave a graphic tracing (a curve) of the 

 contraction. In a similar manner the commutator was arranged 

 so as to excite the point a ; this gave a second curve, situated 

 a little in front of the first. The distance from the beginning 

 of the two curves compared to the length of the nerve enabled 

 the experimenters to determine the rate with which the 

 excitation was transmitted. They then marked on the cylinder 

 the part where the nerve was excited. For this purpose, the 

 commutator was closed in such a manner as to permit excita- 

 tion, when contact was made, between the two needles. At 

 this moment a contraction was produced, which inscribes itself 



