. Elements of human physiology. Physiology. THE CONTRACTILE TISSUES 123 needle of the galvanometer returns to zero of the scale. This adjustment is first made, using only —^ of the total current, and then by means of the shunt, Y^, ^, and finally the whole current is thrown into the galvanometer. If this precaution be not taken, much too large a current may in the first case be sent through the galvanometer, to the detriment of the instrument. If we know the difference of potential between the two ends of the wire. the proportion ^ ^ will give us the of the demarcation current. ab The g
. Elements of human physiology. Physiology. THE CONTRACTILE TISSUES 123 needle of the galvanometer returns to zero of the scale. This adjustment is first made, using only —^ of the total current, and then by means of the shunt, Y^, ^, and finally the whole current is thrown into the galvanometer. If this precaution be not taken, much too large a current may in the first case be sent through the galvanometer, to the detriment of the instrument. If we know the difference of potential between the two ends of the wire. the proportion ^ ^ will give us the of the demarcation current. ab The galvanometer needle having by compensation been brought to zero, stimu- lation of the nerve at (e) by interrupted currents causes the needle to swing at once in the opposite direction to the first variation. This swing is the measure of the negative variation or current of action. The negative variation may also be observed, by means of a YGYy lightly moving galvanometer, to accompany a single twitch of the muscle. It has been found to occupy about .j^ second, and to occur immediately after stimulation. Since a delicate galvanometer takes some seconds to come to rest when a current is sent through it, the whole of the variation is over at a time when the magnet has hardly commenced its swing. Hence to analyse more fully the electrical change accompanying each separate twitch of the muscle, recourse must be had to other methods than the direct galvanometric method. For this purpose we generally use an instrument called the rheotome, by which we can connect the electrodes on the muscle with the galvanometer at. Diagram of rheotome (Hermann). varying intervals after stimulation, and by observing the galvanometer readings at each j^ second after stimulation, can map out the exact course of the current of action. The instrument is represented diagrammatically in Fig. 55. By means of a clock or motor the rod, a b, is made to rotate at any required rate round a vertical axis at its c
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