THE EFFECT OF DRUGS 459 



nerves are regenerated and motor connections reestablished. If such stim- 

 ulation is not applied, the muscle degenerates from disuse and loses its irri- 

 tability often before the nerves regenerate. 



The Effect of Drugs. Drugs affect the irritability of muscle, some 

 augmenting, others depressing it. Voluntary muscle, which does not ordina- 

 rily contract except when stimulated, can be made so irritable by certain 

 salts that it contracts automatically like heart muscle, and the converse. 

 Ether, chloroform, etc., anesthetize muscle just as they do nerve, suppressing 

 both irritability and contractility. Suprarenal extract increases the ampli- 

 tude of contraction, as do also caffeine, digitalis, nicotine, and others. Ver- 

 atrine is well known greatly to prolong the relaxation phase of the simple 

 contraction without materially affecting the contraction phase, or the latent 

 period. 



TETANIC AND VOLUNTARY MUSCULAR CONTRACTIONS. 



Effect of Rate of Stimulation. If we stimulate the muscle-nerve 

 preparation with two induction shocks, one immediately after the other, when 

 the point of stimulation of the second one corresponds to the crest of the con- 

 traction of the first, a second curve, figure 330, will occur, which will commence 

 near the highest point of the first and will rise nearly as much higher, so that 

 the sum of the height of the two curves almost exactly equals twice the 

 height of the first. This phenomenon is called summation. If a third 



FIG. 330. Tracing of a Double Muscle-Curve. To be read from left to right. While the 

 muscle was engaged in the first contraction (whose complete course, had nothing intervened, is 

 indicated by the dotted line), a second induction shock was thrown in, at such a time that the 

 second contraction began just as the first was beginning to decline. The second curve is seen to 

 start from the first, as does the first from the base line. (M. Foster.) 



and fourth shock be passed, a similar effect will ensue, and curves one 

 above the other will be traced, the third being slightly lower than the 

 second, and the fourth than the third. If a continuous series of shocks 

 occur, however, the lever after a time ceases to rise any further, and the con- 

 traction, which has reached its maximum, is maintained. The condition 

 which ensues is called Tetanus. A tetanus is really a summation of contrac- 



