Licence: Public Domain Mark
Credit: A text book of physiology / by William Rutherford. Source: Wellcome Collection.
Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.
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![pair of insulated wires or plates of copper or platinum. But when it is important to avoid the currents generated by the contact of moist tissues with copper and even with platinum, non-;polarisabU electrodes are re- quired. The non-jwlai-isahle electrodes of Doiiders are often used. They consist of a ])air of glass tubes (Fig. 114, h h') closed at their lower ends with plugs of sculptor's clay moistened with water or verj' dilute solution of sodium chloride. The contact of nerve or muscle with clay so pre- pared yields no electricity. The tubes are partially tilled with a saturated solution of pure zinc sulphate, into which there dips a rod of pure amalgamated zinc, to which a wire of the circuit is at- tached. This arrangement is needed, because the contact of a copper wire with the moist clay generates a current. Unfortu- nately the solution diffuses through the clay, and renders it u.seless in the course of a day. muscle and nerve. It consists of a vulcanite table with a The non-polarisable electrodes glass shade. The air is kept moist by wet blotting paper (a); of Fleischl are preferable to the 6 V, Non-polarisable electrodes fixed to flexible leaden rods; above. They consist of a pair of c, paraffin block with a muscle on it. small tubes closed at the lower ends with plaster of Paris, in the centre of which a small camel-hair brash is included. One half of the brush projects free from the plaster plug, and is the terminal to which the tissue is applied. It is moistened with saliva, or dilute sodium chloride solution (1 per cent). The other arrangements in the tubes are the same as in Bonders' electrodes. The electrical circuit is closed and opened by various forms of elec- trical keys. Those most convenient for experiments on stimulation of the tissues are (1) a key with a platinum contact, such as the Morse telegraph key; and (2) a mercurial key where the contact is made by dipping a copper, or better, a platinum wire into mercury. The mercurial key is to be preferred in all cases where successive shocks of uniform in- tensity are desirable. For this purpose the surface of the mercury must be kept perfectly clean, and the production of sparks prevented. This object is best attained by the mercurial key devised by Kronecker and Stirling (Op. 1, ix. 318), in which the surface of the mercury is continu- ally washed by a stream of dilute alcohol. a. Stimulation by Galvanic Electricity. — In physiological experiments the voltaic elements of Grove, Daniell, and L6clanch6 are commonly used. Grove's elements of small dimensions are the most convenient. When a galvanic stream of sufficient intensity is sent along the nerve of a physiological limb (Fig. 1), the nerve is stimulated and the muscle thrown into contraction at the closure or opening, or at both the closure and the opening of the circuit. There is no stimulation of the nerve during the interval of flow, provided the intensity of the current be constant; but if the intensity be varied suddenly and sufficiently, the nerve is excited. Analogous results are obtained when the sensory fibres of a nerve-trunk are made the subject of experiment. It may therefore be formulated as a law that a galvanic stream stimulates a nerve-trunk only](https://iiif.wellcomecollection.org/image/b21981747_0155.jp2/full/800%2C/0/default.jpg)
