. The elements of physiological physics: an outline of the elementary facts, principles, and methods of physics; and their applications in physiology. Biophysics. Chap. XL] NEC A TI l''E VA RIA TION. I I 9 deflected by the muscle current ; the key of the primary circuit is then closed, and that of the short circuit opened, so that the muscle is tetanised, when the needle will be found to swing back, sometimes almost to zero. On closing the short circuit the muscle ceases to be stimulated, tetanus disappears, and the needle is again deflected, but not so much as before. Care must be taken that
. The elements of physiological physics: an outline of the elementary facts, principles, and methods of physics; and their applications in physiology. Biophysics. Chap. XL] NEC A TI l''E VA RIA TION. I I 9 deflected by the muscle current ; the key of the primary circuit is then closed, and that of the short circuit opened, so that the muscle is tetanised, when the needle will be found to swing back, sometimes almost to zero. On closing the short circuit the muscle ceases to be stimulated, tetanus disappears, and the needle is again deflected, but not so much as before. Care must be taken that the induction coil is so far away from the galvanometer that the opening and closing of its circuit have no effect on the needle, and also that the position of the muscle is not shifted during contraction. A good way to obtain the latter with certainty is to use the electrodes (Fig. 59), and to make the points press accurately on the centre of cross-section and longitudinal surface. To prove that the tetanising current does not gain access to the galvanometer circuit and cause an error, tie a piece of wet silk thread round the nerve below the exciting electrodes, and, everything else being unmoved, send on the tetanising stream as before. The continuity of the nerve for nervous stimulation has been destroyed ; no tetanus occurs in the muscle, and no negative variation arises. The continuity of the nerve for electrical cur- rents is, however, still unimpaired, so that the negative variation is not due to any diffusion of electrical currents from the exciting electrodes. Fio. 61 The electric currents of nerves neWe ar-. may be demonstrated in a similar way. thege Here also it will be found convenient to poiarisabie use the tube electrodes. The nerve (a long trodes. piece of the sciatic nerve) may be laid over one clay point turned up into a hook, and the two depending ends made to touch, by their transverse section, the clay point of the second electrode, placed. Please note that
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