424
H. P, BOWBITCH AND J. W. WA BRUN.
edges. A pawl on the axis of the cylinder can he adjusted so as to
fit into a slot in either of these discs, which thus communicate to the
cylinder a movement of rotation at the rate of once in one hour or
once in twelve hours, according to the disc to which it is attached.
On the surface of this cylinder are recorded not only the variations
in the size of the leg, as shown by the movements of the writing
lever of the Marey’s drum already described, but also the moment
at which an irritation is applied to the sciatic nerve. This is effected
by means of the key shown in Plate XIII. C and also diagrammatically-
at fig. 1, I. As is evident from the diagram, this key when closed
short-circuits the current from the secondary coil of an induction
apparatus, so that a rise of the pen connected with it indicates the
opening of this circuit and the consequent transmission of the current
to the nerve.
The Electric Interrupter.
In order to produce a series of induction shocks, the intervals
between which could be varied at will, the apparatus employed in the
first experiments was one constructed in the laboratory on the principle
of Bernstein’s acoustic current breaker. It consisted of a steel rod
bearing a platinum point, which dipped into mercury and was kept in
vibration by an electro-magnet. By clamping it in various positions
the rate of vibration could be varied from 4 to 80 in 1". For slower
rates the vibrating steel rod was replaced by a long spiral spring
hanging vertically and supporting one end of a rigid steel rod, the other
end of which rested upon a knife-edge. To this rod weights could be
attached in different positions, thus varying the tension of the spiral
spring and its consequent rate of vibration. The vibrations were main¬
tained in the usual way by means of an electro-magnet placed above
the steel rod, and a platinum point dipping into mercury below. With
this apparatus rates of vibration as slow as one in 1" could be obtained.
For still slower rates the current was interrupted by means of an
attachment to the hammer of a metronome.
All these instruments have the common defect that both opening
and closing induction shocks are produced and sent through the nerve.
It is, however, necessary in a careful study of the effect of variations
in the rate of stimulation of a nerve to employ stimuli of perfectly
uniform character, varying only in intensity and rate. In other words a
series either of opening or of closing induction shocks should be used.