CONTRACTION IN INVOLUNTARY MUSCLE AND IN CILIA 495 



During rigor mortis the sarcolactic acid is increased, and in addition 

 myosin is formed. 



From these data it is assumed that the processes which take place in 

 resting and active muscles are somewhat different, at any rate in degree, 

 from actively contracting muscle. Also, there are obtained an increased 

 amount of heat and mechanical work; potential energy is converted into 

 kinetic energy. 



THE TYPE OF CONTRACTION IN INVOLUNTARY 

 MUSCLE AND IN CILIA. 



Cardiac Muscle. Some detail concerning the action of cardiac muscle 

 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 

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

 less physiologically isolated from each other, and one fiber may contract with- 

 out 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 con- 

 traction there is a certain interval between the beginning and the crest of the 

 contraction, 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 o . 7 to o . 8 of a second. In the terrapin's 

 cardiac muscle the time of a contraction is over a second, but in the warm- 



