220 



PRACTICAL PHYSIOLOGY. 



[XL 



second curve is superposed on the first, and the height of the com- 

 pound is greater than the original muscle-curve (fig. 141, 3). 



(iv.) Apply the second stimulus within the latent period of the 

 first contraction. There is practically no alteration in the height 

 of the curve (fig. 141, 3). 



2. Tetanus. A tetanising current may be obtained by Neef's 

 hammer, or by means of -a vibrating rod. Apparatus. Daniell's 



FIG. 142. Scheme of arrangement for Tetanus. VS. Vibrating spring ; M. Cup for 

 mercury. Other letters as before. 



cell, five wires, flat spring, cup of mercury in a wooden stand, 

 induction coil, Du Bois key, drum moving at the rate of 5 cm. 

 per second, i .e., the cylinder moves once round in ten seconds, 

 crank-myograph. 



(a.) Arrange the ex-periment as in fig. 142 ; the induction coil 

 for single shocks, short-circuiting the secondary circuit. Place in 

 the primary circuit the flat metallic spring, held in a clamp. One end 

 of the spring has a needle fixed at right angles to it, which dips into 

 a cup of mercury. The needle hangs just above the mercury cup 



FIG. 143. Curves of incomplete and almost complete Tetanus. 



when the spring is at rest, but dips in and out of the mercury when 

 it vibrates. The clamped end of the spring is connected with the 

 battery, while the mercury cup is connected with the induction 

 coil. Cover the mercury with alcohol and water (i 13), to prevent 

 oxidation, and to keep the resistance more uniform. Select a 

 strength of shock which gives response only at break, thus eliminat- 

 ing the make shock. 



