CONTRACTION IN INVOLUNTARY MUSCLE AND IN CILIA 465 



sible that the clotting of myosinogen which is supposed to occur during con- 

 traction is not of the same intensity or extent as that which occurs post mortem. 

 The relation of the hypothetical inogen to the rest of the muscle fiber is unde- 

 termined. It may be that the inogen is formed by the activity of the muscle- 

 protoplasm and stored up within itself, and that during rest of muscle it is 

 gradually used up, whereas in activity it is suddenly and explosively decom- 

 posed. In the rest of the fiber the nitrogenous metabolism continues much 

 the same during activity. 



THE TYPE OF CONTRACTION IN INVOLUNTARY 

 MUSCLE AND IN CILIA. 



Cardiac Muscle. Some detail concerning the action of cardiac mus- 

 cle has already been given in connection with the chapter on Circulation. 

 As compared with the activity of skeletal muscle, cardiac muscle differs 

 most strikingly in that it is automatic. A strip of heart muscle taken from 

 any part of the heart, under proper conditions, gives off a series of contrac- 

 tions, whether it receives any special stimulus or not, whereas we have just 

 found that skeletal muscle under similar conditions remains quiet unless stimu- 

 lated in some special way. The fibers of skeletal muscle are more or less 

 physiologically isolated from each other, and one fiber may contract without 

 involving contractions of the others. Cardiac muscle, on the other hand, 

 when stimulated at any point conducts the change produced throughout the 

 continuity of the mass. Cardiac muscle contractions are influenced by 

 tension, temperature, fatigue, etc., apparently, in the same way as skeletal 

 muscle. 



When the contraction occurs it is always maximal. The actual am- 

 plitude of the contraction is dependent on the condition of nutrition of the 

 cardiac muscle. If the contractions are at a rapid rate they will be relatively 

 of less amplitude. If an extra contraction is induced in an automatic series, 

 so that the interval between two contractions is similar, then the amplitude 

 will be correspondingly reduced. Such an extra contraction is followed by 

 a delayed automatic contraction, the phenomenon of compensatory pause. 

 The contractions in cardiac muscle are simple contractions. In fact, it is said 

 to be impossible to produce a tetanus except in certain invertebrate hearts. 

 This possibility depends upon the fact that during the time of a single contrac- 

 tion there is a certain interval between the beginning and the crest of the con- 

 traction, figure 174, in which the heart muscle is not irritable. This is known 

 as the refractory phase. 



The duration of the contraction of heart muscle is much greater than 

 the contraction of skeletal muscle. The total time of a contraction in a frog's 

 gastrocnemius is o.i of a second, while the time of a contraction of the ven- 

 tricle in the same animal is at least 0.7 to 0.8 of a second. In the terrapin's 

 30 



