248 



PRACTICAL PHYSIOLOGY. 



[XLIX. 



the current so obtained is not weak enough. The simple rheochord 

 should then be used (p. 163). 



(c.) Pull the slider farther 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, contraction 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 may be required to work out all the details of the law. Instead 

 of reversing the commutator after testing the effect of an alteration of the 

 direction of the current, the student may use one preparation to test at 

 intervals the effect of weak, medium, and strong currents when the current 

 is ascending, and a second preparation to test the results with currents of 

 varying intensity when the current is descending. The results may be 

 tabulated as follows : Rarest; C = contraction : 



2. Electrotonic Variation of the Electro-motivity. 



(a.) Arrange a long nerve on N.P. electrodes, as for determining its demar- 

 cation-current. Place the free end of the nerve on a pair ot N.P. electrodes 

 the polarising current arranged as in Lesson XLVIII., so that the current 

 can be made ascending or descending. 



(b.) Take the deflection of the galvanometer needle or demarcation-current 

 when the polarising current is shut off. Throw in a descending polarising 

 current, and observe that the spot of light travels towards zero. Reverse the 

 commutator and throw in an ascending current, the spot of light shows a 

 greater positive variation than before. From this we conclude that kathe- 

 Icctrotonus diminishes the, eledro-motivity . while aiielectrotonns increases it. 

 In the extra-polar kathodic region an electrotonic current appears when the 

 polarising current is closed. It has the same direction as the polarising 

 current. In the anodic region the direction is also that of the polarising 

 current ; but the electrotonic current is stronger than the kathodic current. 

 If a demarcation-current exists already, the electrotonic currents are super- 

 posed on it. 



