POHL'S COMMUTATOR. 161 



(i.) Set Neefs hammer going, and turn the handle 

 of the commutator so that the current passes through both 

 nerves; only the non-poisoned leg (N P) contracts. 



(ii.) Reverse the handle and pass the current through 

 both muscles, both contract. 



(iii.) Push the secondary spiral far away from the 

 primary, and pass the current through both inuscles. At 

 first, if the spirals be sufficiently far apart, there is no 

 contraction in either muscle. Gradually push up the 

 secondary spiral, and notice on doing so that the non- 

 poisoned limb contracts first, and that on continuing to push 

 up the secondary spiral, both muscles ultimately contract 

 (Rosenthal's Modification). 



4. Pohl's Commutator (Fig. 59) is used for sending a current 

 along two different pairs of wires, or for reversing the direction 

 of the current in a pair of wires. It con- 

 sists of a round or square wooden or ebonite 



block with six cups, each in connection with 

 a binding screw. Between two of these 

 stretches a bridge insulated in the middle. 

 The battery wires are always attached to the 

 cups connected with this (1 and 2). When 

 it is used to pass a current through different F - 59 _ Pohl s Com . 

 wires, the cross-bars are removed and wires Siutator with cross- 

 are attached to all six cups, 3 and 4, 5 and 6. bars in. 

 On turning the bridge to one side or other 

 the current is sent through one or other pair of wires. To 

 reverse the direction of a current, only one pair of wires, beside 

 the battery wires, is attached to the mercury cups e.g., to 3 

 and 4, or 5 and 6, the cross-bars remaining in. 



5. The student, if he desires, can prepare two nerve-muscle 

 preparations, and dip the nerve of one (A) and the muscle of the 

 other (B) into a solution of curara in two watch-glasses. On 

 stimulating the nerve of A its muscle contracts ; on stimulating 

 the nerve of B its muscle does not contract, but the muscle 

 contracts when it is stimulated directly. In A, although the 

 poison is applied directly to the nerve trunk, the nerve is not 

 paralysed. 



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