6 54 



A MANUAL OF PHYSIOLOGY 



needle of the galvanometer) moves in such a direction as to indicate 

 that in the extrapolar regions parts of the nerve nearer to the anode 

 are positive to parts more remote, and parts nearer to the kathode are 

 negative to parts more remote. 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. 639). 



8. 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 

 gastrocnemius 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 

 electrotonic current set up in the peroneal 

 nerve passing through the fibres for the 

 gastrocnemius, where they lie side by side in the trunk of the sciatic. 



9. Alterations in Excitability and Conductivity produced in 

 Nerve by the Passage of a Voltaic Current through it. (a) Set 

 up two pairs of unpolarizable electrodes in the moist chamber. 



FIG. 222. PARADOXI- 

 CAL CONTRACTION. 



FIG. 223. ARRANGEMENT FOR SHOWING CHANGES OF EXCITABILITY 

 PRODUCED BY THE VOLTAIC Ct'KRENT. 



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



Connect a battery of two or three Daniell cells, arranged in series 

 through a simple key w.ih the side-cups of a Pohl's commutator with 



