XXXIII.] INDEPENDENT MUSCULAR EXCITABILITY. 193 



5. PohFs Commutator (fig 113) 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 consists 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 

 (i and 2). When it is used to pass a current 

 through different wires, the cross-bars are 

 removed and wires are attached to all six cups, 

 3 and 4, 5 and 6. 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, besides the battery wires, is attached to the mercury cups, 

 e.g., 10 $ and 4, or 5 and 6, the cross-bars remaining in. 



ADDITIONAL EXERCISES. 



6. Curare and Rosenthal's Modification. 



(a.) Prepare a frog as in the previous experiment, ligature the left leg- all 

 except the sciatic nerve and inject curare. After complete paralysis occurs, 

 dissect out both legs with the nerves attached. Attach straw flags (NP and P) 

 of different colours to the toes of both legs by pins, and fix both femora in 

 muscle -forceps (F) with the gastrocnemii uppermost (fig. 114). Place the 

 nerves (N) on the platinum points of Du Bois-Reymond's electrodes (fig. 98). 



(b.) Arrange the induction apparatus as in fig. 114, connecting the 

 terminals of the secondary coil with the piers of a Pohl's commutator (fig. 113) 

 without cross-bars (H). Two other wires pass from two other binding screws 

 of the commutator to the electrodes (N), while two thin wires pass from the 

 other two binding screws (C), and their other ends are pushed through the 

 gastrocnemii muscles. The commutator enables the tetanising currents to be 

 passed either through both nerves or both muscles. It is more convenient 

 if the secondary circuit have a key, so that it may be short-circuited when 

 desired. 



(i.) .SetNeef's hammer going, and turn the handle of the commutator so 

 that the current passes through both nerve* ; only the non -poisoned leg (NP) 

 contracts. 



(ii. ) Reverse the handle and pass the current through both muscles ; both 

 contract. 



(iii.) Rosenthal's Modification. Push the secondary spiral faraway from 

 the primary, and pass the current through both muscles. At first, if the coils 

 be sufficiently far apart, there is no contraction in either muscle. Gradually 

 push up the secondary coil, and notice on doing so that the non-poisoned 

 limb contracts first, and that, on continuing to push up the secondary coil, 

 both muscles ultimately contract. 



7. Action of Curare Bernard's Method. Prepare two nerve-muscle pre- 

 parations, and dip the nerve of one (A) and the muscle of the other (B) into a 



