332 



PHYSIOLOGY OF THE INVERTEBRATA. 



exciting the nerve at a point near the muscle, and noting the 

 moment of excitation and the moment of contraction, one is 

 able to ascertain the time which, elapses between the two 

 phenomena : the same experiment is then repeated on a point 

 of the nerve further from the muscle. The difference in the 

 time observed in these two experiments — that is to say, the 



Fig. 66.— Apparatus for Studying the Transmission of Motor 

 Excitation in the Nerve of the Chela. 



M = myograph carrying claw of Homarus. s = style attached to the dactyl- 

 opodite. r = elastic spring which holds the dactylopodite. a — pair of 



electrodes. d = another pair of electrodes. C — commutator. P = battery. 

 E = registration cylinder. BB' = the two coils. A = steel needles for closing 

 the circuit at each revolution of the cylinder. 



lapse of time between the second contraction and the first — 



gives the time employed by the motor excitation to run the 



distance between the two excited points. Knowing this 



distance, one can calculate the rate of transmission. 



Fredericq and Vandevelde exposed the nerve (in a living 



lobster) which leads to the claw by two openings. A style* 



* The style used was that of Dr. Marey, the distinguished Professor of 

 Experimental Physiology in the College of France. 



