130 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



It is hard to think of one and the same kind of muscle substance con- 

 tracting and relaxing quickly at the same time that it is continuously con- 

 tracted, and the attempt has been made to explain the phenomenon on the 

 assumption that every muscle-fibre contains two kinds of contractile sub- 

 stance, and that the anisotropic fibrillary structures of the fibre are capable 

 of rapid contractions and the isotropic sarcoplasina of* slow contractions. 

 According to this, the first quick rise of the contraction of the veratrinized 

 muscle, etc., is due to the anisotropic substance, and the prolonged after- 

 contraction to the sarcoplasma. This explanation offers much that is satis- 

 factory, but can scarcely be accepted until we are sure that anisotropic and 

 isotropic substances are capable of independent contractions. 



The prolonged contraction of a muscle treated by veratria is an active 

 process, and not merely the result of a change in its physical condition, such 

 as an increase in elasticity. This is shown by the fact that during the stage 

 of contracture the muscle liberates more heat than when at rest. The heat 

 developed during a single veratria contraction may be as much as is given 

 off by a normal muscle excited to tetanus for two seconds. 1 The fact that 

 the prolonged contraction of the veratrinized muscle disappears on etheriza- 

 tion, and returns as the effect of the ether passes off, also favors the view 

 that it is dependent on physiological activity of the muscle protoplasm. 2 



This conclusion is likewise indicated by the observation that the prolonga- 

 tion of the contraction is most marked at a moderate temperature, and fails 

 at very low or very high temperatures. 3 It has sometimes been thought that 

 it was an expression of fatigue, but this can hardly be the case, because it is 

 seen when the rate of the rise and the height of the curve of contraction are 

 normal, and it ceases in the case of the veratrinized muscle if the muscle is 

 worked for a time, and reappears when it has become rested. Moreover, 

 veratria in small doses strengthens the contractions of fatigued muscle and 

 increases its irritability, so that it responds to smaller stimuli by more work. 

 It would appear that we may conclude that the contracture of the veratrin- 

 ized muscle, like that of the normal muscle, is a true contraction process, but 

 that we must await further evidence before deciding as to the exact nature of 

 the contractu re. 



Effect of Fatigue. — If a muscle be excited to contraction by frequent exci- 

 tations, its irritability for a time will be increased, the contractions will 

 become stronger, higher, and more prolonged. If a muscle be excited to 

 contraction at too slow a rate to cause an increase of irritability, it will 

 gradually fatigue, and, as it does so, its contractions will become weaker, 

 lower, and more prolonged. The prolongation of the contractions seen in 

 these two cases is probably due to quite different causes. 



In tin' fir.-t experiment it was a true contraction process; in the second it 



1 Kick and Boehtn : Verhand. der physikal.-med. Oesellschaft in Wiirzburg, 1872, Bd. iii., N. 

 ]•'., S. 198. 



2 Locke: Journal of Experimental Medicine, 1896, vol. i. p. 630. 

 BruntOE and Cash: Journal of Physiology, 1883, vol. iv. p. 237. 



