PRACTICAL EXERCISES 



741 



The direction of movement of the mercury (or galvanometer needle) 

 must be made out first for one direction of the polarizing current. 

 Then the latter must be reversed, and the movement of the mercury 

 (or needle) on closing it again noted (p. 728).] 



7. Paradoxical Contraction. Pith a frog (brain and cord). Dis- 

 sect out the sciatic nerve down to the point where it splits into two 

 divisions, one for the gastrocnemius b, and the 



other for the peroneal muscles a. Divide the 

 peroneal branch as low down as possible, and 

 make a muscle-nerve preparation in the usual 

 way. Lay the central end of the peroneal 

 nerve on electrodes connected through a simple 

 key with a battery of two Daniell cells. When 

 the peroneal nerve is stimulated the gastroc- 

 nemius muscle contracts. This result is not 

 due to the current of action, for it is not 

 obtained with mechanical stimulation of the 

 nerve ; but it is not the result of an escape of 

 current, for if the peroneal nerve be ligatured 

 between the point of stimulation and the 

 bifurcation, no contraction is obtained. The 

 contraction is really due to a part of the electro- 

 tonic current set up in the peroneal nerve 

 passing through the fibres for the gastrocne- 

 mius, where they lie '{side by side in the 

 trunk of the sciatic. 



8. Alterations in Excitability and Conduc- 

 tivity produced in Nerve by the Passage of a Voltaic Current through 

 it. (a) Set up two pairs of unpolarizable electrodes in the moist 

 chamber. Connect a battery of two or three Daniell cells, arranged 

 in series through a simple key with the side-cups of a Pohl's commu- 

 tator with cross- wires in. Connect the commutator to one pair of the 



FIG. 295. PARADOXI- 

 CAL CONTRACTION. 



FIG. 296. ARRANGEMENT FOR SHOWING CHANGES OF EXCITABILITY 



PRODUCED BY THE VOLTAIC CURRENT. 



M, muscle ; N, nerve ; E 1? E. 2 , electrodes connected with secondary coil S ; E 3 , 

 E 4 , unpolarizable electrodes connected with Pohl's commutator (with cross- 

 wires) C ; B', 'polarizing' battery; B, 'stimulating' battery in primary circuit 

 P ; K, K", simple keys ; K', short-circuiting key. 



unpolarizable electrodes (' the polarizing electrodes '), as in Fig. 296. 

 The other pair of unpolarizable electrodes (' the stimulating elec- 

 trodes ') are to be connected through a short-circuiting key with the 

 secondary of an induction machine arranged for tetanus. A single 

 Daniell is put in the primary coil. Pith a frog (brain and cord), make 



