XLIX.] 



PFLUGEKS LAW OF CONTRACTION. 



247 



4. Conductivity is impaired in the Intra-Polar Region. -Arrange the 

 experiment as in 3, but place the salt on the nerve as far as possible from the 

 muscle. When the salt causes tetanic spasms, close the current through the 

 electrodes, and whether this current be ascending or descending, the spasms 

 cease, because the excitatory change is " blocked " in the intra-polar area. 



LESSON XLIX. 



PPLUGER'S LAW OP CONTRACTION— ELECTRO- 

 TONIC VARIATION OP THE ELECTRO- 

 MOTIVITY— RITTER'S TETANUS. 



1. Pfliiger's Law of Contraction. — Apparatus. — Several 

 Daniell or small Grove cells, commutator with cross-bars, Du 

 Bois and Hg-key, rheochord, N.P. electrodes, moist chamber, wires, 

 recording apparatus. 



(a.) Arrange the apparatus as in the scheme (fig. 173). Connect 

 two Daniell or small Grove cells to a Pohl's commutator ivith cross- 



Fia. 173.— Scheme for Pfluger s Law. R. Rheochord ; B. Battery ; C. Commutator ; 

 E. Mercury key ; K'. Du Bois key ; E. N.P. Electrodes ; iV. Nerve. 



barSj and introduce a mercury key (K) into the circuit : connect 

 the commutator with the rheochord (R). O-vunect the rheochord 

 with N.P. electrodes, introducing a short-circuiting key. Fix to a 

 recording lever a nerve-muscle preparation — with a long nerve — in 

 the moist chamber, and lay the nerve over the electrodes. 



{b.) Begin with all the plugs in position in the rheochord and 

 the slider hard up to the brass blocks. Place the commutator 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. Sometimes 



