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EXPERIMENTAL PHYSIOLOGY. 



(a.) Arrange the apparatus as in the scheme (Fig. 92). 

 Take two Daniell or small Grove cells, connect them to a 

 Pohl's commutator with cross-bars, and introduce a Morse 

 or mercury key (K) into the circuit; connect the commutator 

 with the rheochord (R). Connect the rheochord with a pair 



Fig. 92. Scheme for Pfluger's Law. R, Rheochord. 



of N.P. electrodes with a short-circuiting key introduced. 

 Fix to a lever a nerve-muscle preparation with a long 

 nerve in the moist chamber, and lay the nerve over the 

 electrodes. 



(6.) Begin with all the plugs in position in the rheochord 

 and the slider hard up to the brass blocks. Place the com- 

 mutator to give an ascending current, make and break the 

 current gradually adjusting the slider until a contraction 

 occurs at make and none at break. Reverse the commutator 

 to get a descending current, make and break, observing 

 again a contraction at make and none at break. This 

 represents the effect of a weak current. 



(c.) Pull the slider further away and remove one or more 

 plugs, until contraction is obtained at make and break, both 

 with an ascending and descending current. This represents 

 the effect of a medium current. 



(d.) Use six small Grove's cells, take out all the plugs from 

 the rheochord, and with the current ascending, contraction 

 occurs at break only while with a descending current, con- 

 traction occurs only at make. This represents the effect of 

 a strong current. Tabulate the results in each case. 



For this experiment very fresh and strong frogs are necessary, 

 and several preparations will be required to work out all the 



