Bauhaus-Universität Weimar

Plethysmographic Experiments on the Vaso-Motor Nerves of the Limbs [From the Journal of Physiology, Vol. VII, Nos. 5 & 6, Offprint]
Bowditch, H. P. J. W. Warren
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.


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