of Voluntary Nervous Action. 191 
Besides these numbers there were many failures, in the last 
series, to answer at all. This last series differs in one respect 
from many others. The periods are not all of the same length; 
and if this should prove to be a rule and not an exception a 
modification of the hypothesis would be necessary. At present 
I am inclined to regard it as accidental, as it is opposed to so 
many well-defined examples. The periods after dinner are 
seen to be lengthened and the whole series drawn out. 
at the grouping is not due to the nerves themselves is 
shown by the fact that the nerves of animals recently kill 
transmit the motor impulse with perfect regularity ; and also 
by the fact that, in the living subject, nerves excited by artifi- 
= means transmit the motor impulse with the same regu- 
arity. 
2d. The muscles, in order to remain in sound health, must 
have periods of rest alternating with periods of activity. Even 
the heart, that keeps the blood in ceaseless motion from the 
earliest dawn of our independent existence till the last closing 
act in life’s drama, rests about one-half the entire time; and it 
certainly appears reasonable to suppose that the brain also has 
its periods of rest. Besides this, it is an established physio- 
logical fact that a muscle during contraction is in a state of 
vibration, giving out a continuous sound like a musical tone. 
According to Helmholtz* the pitch of the fundamental ton 
vibration varies somewhat, but Helmholtz and two other 
_ 3d. All of the simple mental or psychological processes, the 
time of which has been measured (and many such measure- 
ments have been made), require a longer interval of time than 
that shown by experiment to pass between two maxima and 
minima, This fact is regarded as specially significant, for, if 
such waxing and waning of nervous activity exists, the sim- 
: Helmholtz, Ueber das Muskelgerausch; Reichert u. du Bois Raymond’s Ar- 
chiv fiir Anatomie, 1864, p. 766. 
Foster, Text-book of Physiology, 2d edition, p. 56. 
