Eight lectures on the signs Eight lectures on the signs of life from their electrical aspect eightlectureson00wall Year: 1903 60 THE SIGNS OF LIFE [lect. We shall study the skin-currents of the frog, of the cat, and of man, also the currents of mucous membranes and the currents of vegetable ' skins.' To-day we shall confine our- selves to the case of the frog, examining: {a) the normal current, {b) the effects of indirect excitation, {c) the effects of direct excitation. § 37. The normal current.—A piece of skin of the back, or indeed of any part of the body, carefully excised and placed betw
Eight lectures on the signs Eight lectures on the signs of life from their electrical aspect eightlectureson00wall Year: 1903 60 THE SIGNS OF LIFE [lect. We shall study the skin-currents of the frog, of the cat, and of man, also the currents of mucous membranes and the currents of vegetable ' skins.' To-day we shall confine our- selves to the case of the frog, examining: {a) the normal current, {b) the effects of indirect excitation, {c) the effects of direct excitation. § 37. The normal current.—A piece of skin of the back, or indeed of any part of the body, carefully excised and placed between unpolarisable electrodes, gives current, directed from outer to inner surface (through the skin). This ' ingoing' ' negative' or centripetal current increases during observation. A piece of skin spread upon a glass plate with a central hole, and placed between the elec- trodes as figured, gives current that reflects the galvanometer spot to your left. And, as you notice, the spot is creeping further to the left. Two re- flections arise from this obser- vation : clearly the current cannot be due to injury of the internal surface, since in that case it would be outgoing; probably the increasing nega- tive current is due to the sub- sidence of what in the case of the eyeball we referred to as the manipulation blaze. It looks as if resting skin were the seat of an ingoing current of rest, and as if the increasing negative deflection occurring on the galvanometer scale were in reality a decreasing positive effect caused by the previous disturbance by manipulation. We shall find confirmation of this view later on, when we have learned that direct mechanical and electrical excitation of the skin gives almost invariably a positive or outgoing electrical effect. Fig. 26.—Frog's skin on a perforated glass or ebonite plate, between unpolar- isable electrodes. The external surface of the skin is uppermost. The arrows signify the direction of ' normal cur- rent ' — ' ingoing,'
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