THE NERVE-MUSCLE PREPARATION 



23 



1. Excitation by galvanic current. Determine that making or 

 breaking the circuit of a battery is a stimulus to the nerve, whereas 

 the passage of the current usually 1 produces no obvious effect. It 

 is proper to use non-polarisable electrodes wherever the current of 

 a galvanic battery is led through a preparation. 



A simple form of battery is furnished by two wires, one of copper, the other 

 of iron or zinc. If these are placed at one end in contact with the moist nerve- 

 muscle preparation, and if the other ends are made to touch one another, the 

 muscle will contract, as in Galvani's original experiment. 



Fleischl's rheonome is designed to show that not only a change of potential 

 but also the suddenness of the change is an important factor in stimulation 

 by a galvanic current. The rheonome consists of a bridge composed of two 

 curved pieces of zinc capable of being rotated about a raised centre. One end 

 of each piece has a terminal attached to it (upper terminals); the other end 

 which should be amalgamated dips into saturated solution of zinc sulphate 

 contained in a circular groove in the wooden base of the instrument. Wires 



FIG. 30. DIAGRAM OP FLEISCHL'S EHEONOME. 



from a battery are brought to opposite sides of the groove and come into 

 connection with the zinc sulphate solution by amalgamated zinc terminals 

 (lower terminals). The upper terminals are brought into relation with a muscle- 

 nerve preparation by other wires, and when the battery circuit is closed part of its 

 current passes to the preparation, part is short-circuited through the zinc sul- 

 phate. The amount of short-circuiting depends upon the position of the rotating 

 bridge. Most current passes through the preparation when the ends of the 

 bridge are opposite the lower or battery terminals. On now rotating the bridge 

 this current is diminished, and it vanishes when the rotation has attained 90 

 of the circle, gradually increasing again as this point is passed. If the rotation 

 is slow, there is no stimulation of the preparation, in spite of the variation in 

 amount of current; but if quick, the muscle-nerve responds to each movement of 

 the bridge. 



2. Excitation by induced current ; Determination of excitability of 

 a nerve. An induction shock is a stimulus, and the break induction 

 shock a far stronger stimulus than the make. Get the minimal 

 effect of each by sliding the secondary coil to the necessary 



1 For exceptions see Chapter XI. 



