PRACTICAL EXERCISES 705 



cluction-machinc must not be confused with the break or make 

 contraction caused by the voltaic current. In the case of the 

 induction machine, the break or make applies merely to what is done 

 in the primary circuit, not to what happens to the current actually 

 passing through the nerve. The current induced in the secondary at 

 make of the primary circuit is, of course, both made and broken in 

 the nerve made when it begins to flow, broken when the flow is 



FIG. 259. CONTRACTIONS CAUSED BY MAKE AND BREAK SHOCKS FROM AN 

 INDUCTION MACHINE. 



M, make, B, break, contractions. The numbers give the distance between the 

 primary and secondary coils in centimetres. 



over ; the shock induced at break of the primary is also made and 

 broken in the nerve. And although make and break of the actual 

 stimulating current come very close together, the real make, here, 

 too, is a stronger stimulus than the real break. 



(6) Repeat (a) with the muscle directly connected to the cell by 

 thin copper wires, or, better, unpolarizable electrodes (p. 625). 



3. Ciliary Motion. Cut away the lower jaw of the same frog, and 



FIG. 260. SIMPLE RHEOCORD^ARRANGED TO SEND A TWIG OF A CURRENT 

 THROUGH A MUSCLE OR NERVE. 



B, battery ; R, rheocord wire (German silver) ; S, slider formed of a short piece 

 of thick indiarubber tubing filled with mercury ; K, spring key ; W, W, wires 

 connected with electrodes. 



place a small piece of cork moistened with physiological salt solution 

 (0-75 per cent.) on the ciliated surface of the mucous membrane 

 covering the roof of the mouth. It will be moved by the cilia down 

 towards the gullet. Lay a small rule, divided into millimetres, over 

 the mucous membrane, and measure with a stop-watch the time the 

 piece of cork takes to travel over 10 millimetres. Then pour salt 

 solution heated to 30 C. on the ciliary surface, rapidly swab with 

 blotting-paper, and repeat the observation. The piece of cork will 



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