246 



PRACTICAL PHYSIOLOGY. 



[XLVIII. 



the rheochord shown in fig. 92. By reversing the commutator the current 

 through the rheochord can be reversed. Then connect one N.P. electrode with 

 one terminal of the rheochord, while the other N.P. is connected with the 

 movable block or slider (S) of the rheochord. 



(c.) Notice which pole is next the muscle according to the position of the 

 commutator and make a mark to guide you. Make a long nerve-muscle and 

 arrange it over the electrodes, attaching the muscle to a recording lever 

 (crank). 



A 



+ 



K 



e a 



i 



Fro. 171.— Scheme of Electrotonic Variation of Excitability in a Nerve. K. Kathode ; 

 A. Anode ; iV, n. Nerve. The cuive above the line indicates increase, and that below 

 the line decrease of excitability. 



{d.) Begin with the slider (S) close up to the zero terminal, and gradually 

 slide it along until, on closing the battery circuit, the muscle responds at make 

 whether the + or - pole is next the muscle, i.e., whether the current is 

 ascending or descending. 



(e.) Open the circuit, place on the nerve near the muscle either a drop 

 of saturated solution of common salt or fine moist crystals of salt. Wait till 

 the salt produces occasional short spasmodic movements of the limb. Close 

 the key, place the - pole next the muscle, at once the limb becomes tetanic 

 owing to the increase of excitability under the influence of the - pole {kath- 

 electrotonus). Open the current, the limb becomes quiescent. 



if.) Open the key, and 

 after a short time, when 

 the spasms reappear, 

 reverse the commutator 

 so that the + pole is next 

 the muscle. Close the 

 current, the limb becomes 

 quiescent, due to the fall 

 of excitability under the 

 influence of "the + pole 

 [aneledrotoniis). Break 

 the current, the muscle 

 becomes tetanic. Thus it 

 is shown that the appear- 

 ance of kathelectrotonus and the disappearance of anelectrotonus are accom- 

 panied by increase of excitability, while the disappearance of kathelectrotonus 

 and the appearance of anelectrotonus are accompanied by diminution of 

 excitability. 



Flo. 172.— Pohl's Commutator with cross-bars, arranged 

 for reversing the direction of a current. 



