During the investigation of the motor
components of unconditioned and conditioned
reflexes a good deal of attention has been paid
to the study of movements of the mouth.
The methods suggested for recording the
movements of the mouth in dogs are based on the
principle of air transmission: the mouth of the
animal is surrounded by a rubber band, and when
yawning movements occur and the mouth is opened
this transmits pressure to a rubber balloon
attached under the lower jaw or to the dorsum of
the nose of the animal.
However, when the amplitude of the movements
is great, for example, during the act of yawning
or in some forms of defensive reflexes, the
pneumatic method is unsound because in such
cases the balloon will always experience the
maximum pressure and the tracing will show the
mouth movements disproportionately.
A silk thread, many times longer than the
circumference of the wide-open mouth of the
animal, is glued at its center to the skin of
the lower jaw of the animal. Both ends of the
thread, passed round the muzzle of the dog on
each side, are taken through the limbs of a
light glass Y-tube into a composite glass tube,
fixed at one end to a cork disc which is glued
to the skin of the dorsum of the nose of the
animal, and at the other end to the cross-beam
of the stand. The composite glass tube is formed
of sections of drawn glass tubing joined
together with rubber connecting tubes. The
length of the tube is sufficient to allow free
change of position of the animal on the
apparatus. The ends of the loop of thread, after
passing through the composite glass tube, are
connected to the recording lever and with each
increase in the circumference of the loop the
pen of the recording lever is deflected
downwards. The return movement of the lever,
which is counterpoised, occurs when the diameter
of the loop is reduced as the animal closes its
mouth. The composite glass tube system
effectively damps any general movements.
Since the length of thread inside the glass
tube at any given moment cannot change
appreciably, the only part of the silk thread
which can affect the position of the recording
arm is the loop around the animal's mouth. Since
this loop is freely extensible to any degree to
which the animal may open its mouth, the
recording will reflect completely the character
of the movements of the mouth.
The method suggested may be used in tracing
the mouth movements with amplitudes from the
very slightest to the maximum.
The outline of the recording and the form of
the tracing are shown in figure.
Mechanical transmission by way of a silk
thread contained in a glass tube is proposed for
registration of mouth movements. Registration of
all forms of mouth movements is possible.