192 
Proceedings of Eoyal Society of Edinburgh. [sess. 
twitcli, both in its motor and in its electrical aspects, have been 
carefully investigated. Further, it is known that if the stimuli come 
after each other with sufficient rapidity, a tetanic state is produced. 
The duration of the stimulus capable of exciting a muscular con- 
traction has also been noted. Thus Koenig, a pupil of Helmholtz, 
found that weak induction currents lasting less than *001 sec. pro- 
duced no effect, but if the duration of the current was lengthened 
to '017 to *018 sec. a contraction resulted. This observation, of 
which I have doubts, leads us to suppose that a current must last, 
when applied to a frog’s sciatic nerve, at least one-thousandth of 
a second to produce an effect on the muscles supplied by the nerve. 
It is of interest to inquire into the action of stimuli on sensory 
nerves. This can readily be done. I introduced into the circuit 
of the primary various interrupters (clock, metronomes, vibrating 
springs, tuning forks), varying in speed from 1 to 200 vibrations 
per second, and the effect on the skin was noted when the fingers 
were dipped into the vessels containing salt solution in which were 
immersed the platinum terminals of the secondary. It was easy 
to discriminate successive stimuli up to about 40 to 50 per second ; 
between 50 and 120 the sensation was a thrill something com- 
parable to the effect of very rapid musical beats causing disagree- 
able roughness; from 120 up to 200 the effect was of a more solid, 
continuous character, and was the (to me) pleasurable thrill pro- 
duced by a series of weak induction shocks. The sensation of 
continuity of stimulation is analogous to tetanus, although it by 
no means follows that the molecular changes occurring in a muscle 
during tetanus are in the least like those happening in a nerve 
centre which is the seat and material substratum of the sensation. 
As I have said, it is an undoubted fact that the sensory nerves 
of the skin can appreciate electrical stimulations corresponding in 
number, rhythm, and intensity to the notes or chords of a compli- 
cated piece of music. But the sensations experienced when we 
listen to music are very complicated. They may, however, be sub- 
divided into various elements. There is pitch of individual tones, 
duration of individual tones, intensity of tone, quality of tone, and, 
in addition, the relationship of tones, or tonality. Again, we com- 
prehend music in proportion to its rhythm. If it is unrhythmic 
it is incoherent; but if it is rhythmic, if there is a systematic 
