ACTION OF POISONS ON MUSCLE. 563 



fact that the muscle, from the moment of stimulation on, must first accumulate 

 so much contractile force as is necessary to raise the weight. The greater the 

 weight the longer is the period of time that must elapse before the act of lifting 

 begins. Finally, a degree of after-loading is reached at which it is no longer 

 possible to raise the weight. This indicates the limit to which the lever-force 

 may operate. 



If a muscle, during contraction, be subjected to a temporary increase in 

 tension, it will be found that a short, quick, and considerable increase in tension 

 immediately diminishes the contraction ; while a more prolonged and slow increase 

 somewhat later increases the contraction. 



The temperature of the muscle also has some influence. The duration of the 

 contractile force diminishes with increasing temperature, increasing with increase 

 in weighting. The rapidity with which the contractile force develops increases 

 with increasing temperature, diminishing with increased weighting. The height 

 to which an unweighted muscle may lift a weight increases with its temperature. 

 A frog's muscle, supplied with circulating blood, exhibits the greatest contraction 

 in response to stimuli at o C. As the temperature rises, the extent of contraction 

 diminishes progressively. 



If the muscle becomes fatigued as a result of repeated stimulation, the stage 

 of latent stimulation becomes longer and the curve remains lower, because the 

 contraction of the muscle is less; while the abscissa becomes longer, because 

 the muscle contracts more slowly (Fig. 194, I). Cooling of a muscle has like 

 effects. Also the muscles of the new-born behave in a similar manner. The con- 

 traction-curve has a flat apex, and is considerably prolonged, especially in the 

 descending limb. 



If the nerve of the muscle is stimulated by the closing or opening of a constant 

 current, the muscular contraction corresponds exactly to that already described. 

 If, however, the current is applied directly to the muscle itself, and is closed 

 and opened, a certain degree of persistent contraction, though often but slight, 

 takes place during the period of closure, so that the curve assumes the form 

 shown in Fig. 194, IV, in which the current was closed at S and opened at O. 



According to Cash and Kronecker, the individual muscles have a special 

 form of contraction-curve. Thus, the omohyoid of the tortoise contracts more 

 rapidly than the pectoral. The flexors of the frog contract more quickly than the 

 extensors. The muscles of tortoises, the adductors of mussels, the muscles of the 

 bat, and the heart contract slowly. The muscles of flying insects contract with 

 great rapidity, those of the fly 350 times, and of the bee 400 times in a second. 

 There are, however, slowly contracting muscles among beetles also, for example 

 in the water-beetle, hydrophilus. 



White muscle-fibers are more irritable, have a shorter latent period, and are 

 more readily fatigued than red fibers ; their contraction -period is shorter. They 

 are therefore more active, and the contraction -wave is propagated more rapidly 

 in them. They also produce more acid and heat during their activity. The red 

 fibers execute protracted, continuous movements; hence, moderate, physiological 

 tetanus. They intermediate the adjustment of the muscular force to the resistance 

 to be overcome. Red fibers, or those. rich in protoplasm, are further present, 

 especially in the continuously active muscles respiratory, masticatory, ocular, 

 and cardiac. The white fibers execute the rapid, single movements. Muscles 

 that contain principally white fibers have a greater lifting capacity and a more 

 marked absolute power in the single contraction, but they are inferior to the red 

 muscles in tetanic contraction. The contraction-curves of a mixed muscle con- 

 taining white and red fibers may exhibit two elevations in the ascending limb, 

 the first being due to the contraction of the active white fibers, and the second 

 to that of the more sluggish red fibers. These are observed especially after the 

 action of veratrin on the muscle-substance. The nerves supplying the white and 

 red muscles also exhibit differences in their irritability. 



Action of Poisons. Small doses of curare, as well as quinin and cocain, in- 

 crease the size of the contractions induced by stimulation of the nerve; larger 

 doses reduce the size to the point of complete paralysis. Suitable, small doses 

 of veratrin likewise increase the size of the contractions, while the stage of re- 

 laxation is conspicuously lengthened. Acids accelerate the relaxation. Veratrin, 

 antiarin, and digitalin in large doses induce such changes in the muscle-substance 

 that the contractions become greatly prolonged and similar to a continuous, 

 tetanic contraction. In muscles poisoned with veratrin or strychnin, the latent stage 

 of contraction is at first shortened, but later lengthened. The gastrocnemius of a 

 frog will contract more rapidly if supplied with circulating blood containing 

 sodium bicarbonate. Kunkel believes that the essential factor in the action of 



