XLIX.] 



PFLtfGER'S LAW OF CONTRACTION. 



249 



3. Bitter's Tetanus. 



(a.} Connect three Darnell's cells with N.P. electrodes, short-circuiting 

 with a Du Bois key. Make a nerve-muscle preparation, and apply the 

 electrodes to the nerve so that the + pole is next the muscle, i.e., the current 

 is ascending in the nerve. Allow the current to circulate in the nerve for 

 some time (usually about five minutes is sufficient), no contraction takes 

 place. Short-circuit, and the muscle becomes tetanic. 



(6.) Divide the nerve between the electrodes, and the tetanus does not 

 cease ; but on dividing it between the + pole and the muscle, the tetanus 

 ceases. Therefore the tetanus is due to some condition at the positive pole, 

 i.e., the stimulation proceeds from the positive pole at break. 



4. Kathodic Stimulus is the more powerful. 



(a.) Let the M. and B. shocks be made approximately equal by the arrange- 

 ment shown in fig. 174. In the secondary circuit place a PohPs commutator 



FIG. 174. Scheme to show that Kathodic Stimulation is the more powerful. 

 M. Commutator ; F. Frog's leg ; c. One electrode. 



K. Key ; 



with cross-bars (R). Place one electrode (c) under the sciatic nerve, and the 

 ocher on another part of the body. 



(b.) Suppose c to be the cathode, select a strength of shock, i.e., distance 

 of secondary from primary coil, so that there is response on breaking the 

 primary current. Reverse the commutator so that c becomes the anode. 

 There is no muscular response at break, but it occurs at make, as c is then 

 the cathode. 



5. Rheochord. of Du Bois-Reymond is used to vary the amount of a 

 constant current applied to a muscle or nerve (fig. 175). It consists of a long 

 box, with German-silver wire of varying length, and whose resistance is 

 accurately graduated stretched upon it. At one end are a series of brass 

 blocks disconnected with each other above, but connected below by a German- 

 silver wire passing round a pin. These blocks, however, may be connected 

 directly by brass plugs. S, S. 2 . . . S 3 . From the blocks i and 2 two platinum 

 wires pass from A to the opposite end of the box (Y), where they are insu 

 lated. Between the wires is a "slider" (L), consisting of two brass cups 

 containing mercury, which slide along the wires. 



