1 88 PRACTICAL PHYSIOLOGY. [XXXII. 



consists of a square ebonite base, with a grooved circular channel in 

 it, and two binding screws, with zinc attached, and bent over so as 

 to dip into the groove, which is filled with a saturated solution of 

 zinc sulphate. A vertical arm, with binding screws attached to two 

 bent strips of zinc, moves on a vertical 

 support. It is a kind of revolving rheo- 

 chord. 



(a.) Connect two or three DaiiielPs cells 

 (copper to zinc) with the binding screws 

 A and B, introducing a Du Bois key in 

 one wire. Attach the electrodes, intro- 

 ducing a Du Bois key to short-circuit them, 

 to the binding screws, C and D. Fill the 

 FlG ' KheoitK?m SCh1 ' 8 groove with a saturated solution of zinc 



sulphate. 



(b.) Arrange the nerve of a nerve-muscle preparation over the 

 electrodes, or simply expose the sciatic nerve of a frog in situ. Pass 

 a constant current through the nerve, observing the usual effects, 

 viz., contraction at make or break, or both, but none when the 

 current is passing. Then suddenly rotate the handle with its two 

 zinc arms ; this is equivalent to a sudden variation of the intensity 

 of the current ; the current, of course, continuing to pass all the 

 time. The muscle suddenly contracts. 



When the two ends of the zinc arc stand as in the fig., i.e., opposite C and 

 D, then, on closing the current, most of the current goes through the zinc arc 

 to the preparation, and only a small part through the zinc sulphate solution 

 from C to D. Thus the muscle contracts according to the direction and 

 intensity of the current, either on closing or opening the key, or at both. 

 Turn the handle so that the zinc arc is vertical to a line joining C and D. 

 There is no current, so that the preparation does not respond either on closing 

 or opening. 



If, while the zinc arc is in this position, the circuit be closed, and the zinc arc 

 suddenly rotated into the position of the line C, D, the muscle contracts, 

 provided in the first experiment a closing, i.e., make, contraction was obtained. 

 If it be rotated slowly then there is no response. Thus one can allow the 

 current to glide or slide into the nerve ("einschleichen") without causing 

 excitation. 



2. Direct and Indirect Stimulation of Muscle. When the 

 stimulus is applied directly to the muscle itself, we have direct 

 stimulation ; but when it is applied to the nerve, and the muscle 

 contracts, this is indirect stimulation of the muscle. 



(i.) Induced Current. (a.) Arrange a nerve-muscle preparation, 

 and an induction machine for single or repeated shocks (Lesson 

 XXXI. 1). 



(b.) Test first the strength of current as measured by the 

 distance between the secondary and primary coils which causes 



