Directions for classwork in experimental physiology / by Sir Edward Sharpey Schafer.

  • Sharpey-Schäfer, E. A. (Edward Albert), Sir, 1850-1935.
Date:
[between 1915 and 1935?]
Catalogue details

Licence: In copyright

Credit: Directions for classwork in experimental physiology / by Sir Edward Sharpey Schafer. Source: Wellcome Collection.

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    the base of the ventricle. If this electrode is the anode, on closing the key it will be observed that the part of the ventricle underneath it does not participate in the contractions, but remains quiescent, and, if the heart be filled with blood, even bulges during general systole : on opening the key this part passes into systole, even during general diastole (physiological rebound). If the current be reversed and the cotton-wool be made the kathode, the reversed effects are obtained. Polar excitation of nerve.—Take a muscle-nerve preparation with as long a nerve as possible and arrange it on the myograph. Non- polarisable electrodes, connected with a constant battery through a mercury key, are placed—the anode in contact with the uppermost end of the nerve, the kathode in contact with the lowermost end, i.e., close to the muscle. Insert an electro magnetic signal into the circuit and cause it to mark on the drum just below the myograph lever. Record two contractions, one produced by closing the mercury key, the other (on a different abscissa) by opening it. Make a time tracing below, and measure exactly the period of latency in each case, i.e., the time elapsing between the current of the electro-magnetic signal and the commencement of rise of the curve. Notice that it is slightly greater as the result of breaking the circuit than on making (by the time taken for the nerve impulse to traverse the length of nerve), since the excitation at breaking is at the anode, i.e., at a point of the nerve furthest from the muscle, whereas on making the excita- tion was at the kathode, i.e., close to the muscle. If, as represented in Fig. 47, an ascending current is used instead of a descending one, the result is complicated by the blocking effect of the constant current on conduction (see p. 51). Thus, on making such an ascending current, if it were a strong one, the excitation being at the kathode, i.e., at the uppermost end of the nerve, and the inter- mediate part of the nerve being at the same moment traversed by the current, this would block the passage of the nerve impulse generated at the kathode, and no contraction would result. Therefore, instead of obtaining a contraction at both make and break, only the break would produce a visible effect under these circumstances. On the other hand, if the constant current is weak, its removal may not be followed by contraction of the muscle, because the breaking of such a current furnishes a smaller excitation than its making.. The varying effects of opening and closing a galvanic current (misnamed “the law of contraction”) are illustrated by an experiment devised by Pfluger. The nerve of a nerve-muscle preparation is placed on non-polarisable electrodes, which are connected with a battery of at least three Daniell cells through a commutator and rheochord : a