184 ANIMAL PHYSIOLOGY. 
tracing, it may be observed that, during that corresponding to 
the former, though it is shortened, it does not remain equally so 
throughout, while during the writing of the third tracing there 
is no variation in its condition appreciable by the eye. What 
has happened is this: The muscle during the condition figured 
in the second tracing has periods of alternating contraction and 
partial relaxation, but during the third case the latter phase 
has been apparently omitted—the muscle remains in continuous 
contraction. In reality this is not the case unless we are mis- 
taken as to the meaning of the muscle-sound. 
The Muscle Tone.—There are a number of experimental facts 
from which important conclusions have been drawn, to which 
attention is now directed: 
1. It has been found that a sound may be heard in a still 
room when one brings the muscles of mastication into action 
by biting hard; or listens over a contracting biceps with a 
stethoscope, etc. 
2. When the wires of a telephone (communicator) are con- 
nected with a muscle, a sound is heard during the contraction 
of the muscle. 
From these facts it was concluded that a muscle when con- 
tracting gives rise to a sound; that fetanus, as the form of con- 
traction we are describing is called, is essentially vibratory in 
character, which seems to answer to the graphic representations 
from a muscle when in tetanic contraction, and is in harmony 
with the case to which we called attention at the commence- 
ment of this subdivision of the subject. The note heard cor- 
responded, in the case of an isolated muscle, to the number of 
stimulations per second; while for muscles made to contract by 
the will the note was always the same, answering to about 
forty vibrations per second; but as forty stimuli are not re- 
quired within this period of time to induce tetanus, it was 
thought that this note was probably the harmonic of a lower 
one answering to twenty vibrations in a second. 
It has been recently shown that a very much smaller num- 
ber of vibrations of the muscle can give rise to an audible 
sound, so that the explanation it would seem must now be 
modified; and it is likely that some peculiarities of the ear 
itself must be taken into the account in the explanation. In 
making the observations referred to above (in 1), the student 
will find it very important to be on his guard against sources 
of error, especially with the use of a stethoscope. 
We may safely conclude that, at all events, most of the mus- 
