XVI. Demonstration : Pfltiger's law of contraction. 



/. Appliances. Du Bois-Reymond rheocord, or simple 

 rheocord; 3 Daniel cells; muscle signal or myograph 

 with moist chamber; 2 Du Bois-Reymond keys; com- 

 mutator; 2 N. P. electrodes. 



2. Preparation. Set up the apparatus with three cells in 

 series, Du B.-R. key as closing key. Commutator with 

 cross-bars, Du B. R. rheocord in short circuit, short-cir 

 cuiting key, the two N P. electrodes clamped in cham 

 ber of myograph. 



_? Operation. Make and mount a gastrocnemius prepara- 

 tion. 



4. Observations. 



(1) Stimulate with make and break of the weakest pos- 

 sible descending current. 



Record results in such a table as that suggested in 

 laboratory lesson XII. 



This table shows what response (contraction or 

 rest) the muscle gives on the making and breaking of 

 the descending current and on the making and break- 

 ing of the ascending current. 



It also shows in a marginal column the gradual in- 

 crease of the strength of the current through gradual 

 increase of resistance in the short-circuiting rheocord. 



(2) Make and break with weak ascending current. If 

 the conditions are typical the muscle will contract on 

 making both ascending and descending current. 



(3) Increase gradually the strength of the electrode cir- 

 cuit, recording results. After a longer or shorter 

 transitional period in which the result will be varied 



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