1882.] on Animal Excitability. 149 



pinching, descriptive anatomy teaches me that tliesc approach the 

 surface sufficiently closely to be within reach of induction currents 

 led through at the skin, at two places, namely, above the collar-bone 

 and at the bend of the elbow. If, by the means I have indicated, I 

 excite the nerve at either of these points, the hand involuntarily 

 pinches, but if I measure the time which elapses in the two cases 

 between the excitation and the muscular response, I find it to be 

 different — the difference being the measure of the time occupied by 

 the excitatory change, from the clavicle to the bend of the arm.* 



The experiment you have seen not only serves to illustrate the 

 resemblance between the propagation of the excitatory state and that 

 of an explosion, but also to exhibit the contrast between them. It is 

 common to all excitable structures, whether of plant or animal, that, 

 provided the intervals are not too short, the excitations can be repeated 

 any number of times without losing their effect, a fact which can only 

 be explained on the hypothesis that in such structures provision 

 exists for the immediate recuperation of lost energy ; or, in other words, 

 that the machinulae which take part in the propagation of the excita- 

 tory disturbance are endowed with the faculty of quickly recovering 

 their original condition, so as to be ready for another excitation. | 



In bringing before you these elementary facts relating to animal 

 excitability, my object is to use them in the comparison we shall 

 shortly have to make between animals and plants in respect of this 

 projDerty. From the last experiment we have learnt, in addition to 

 the fact which it was sjoecially intended to illustrate, that the sudden 

 change of form of a muscle which we call its contraction is of such a 

 nature that the structure shortens in one direction only, and that it 

 gains in thickness in the same proportion that it diminishes in 

 length. The experiment further illustrates the fact that a muscle 

 does not contract of itself, but that it undergoes this change only 

 when it is directly or mediately excited. Bearing these facts in 

 mind, I would ask your attention to some further characteristics of 



* The measurement is effected by recording the muscular action on a 

 blackened glass plate, which is so fixed to a pendulum that its surface is parallel 

 with the plane of oscillation. The pendulum is allowed to make a single 

 swing from right to left, and in doing so strikes a trigger, the effect of which is to 

 excite the nerve by an induction shock at one or other of the points indicated. 

 Two experiments were made at the lecture in immediate succession, in one of 

 which the nerve was excited at the more distant, in the other at the nearer point. 

 The difference of time between the two records, calculated from the distance 

 from each other of the two tracings, was about -^l of a second. As the distance 

 w^as about 13 inches, this gives about 66 metres per second as the rate of trans- 

 mission. This result was probably not far from the truth, but the reader will 

 understand that a measurement made under the conditions of a lecture experi- 

 ment could not be relied on. 



t The extreme shortness of the intoL-val of time between successive excitations 

 of muscle was illustrated by an experiment in which the rheoscopic limb of a 

 frog was kept in repeated spasmodic action by the voice of the lecturer acting on 

 a telephone of which the wires were in contact with the nerve. 



