GENERAL PHYSIOLOGY OF MUSCLE-TISSUE. 



79 



maximum value (Fig. 25). The rate at which the muscle is 

 stimulated with a given stimulus will also influence the character 

 of the contraction process. If the intervals between the successive 

 stimulations be such as permit the muscle to recover from the 

 effects of the contraction, it may contract as many as a thousand 

 times without showing any particular variation from the normal 

 form. In the earlier period of stimulation there is apparently 

 a decrease in the irritability and consequently in the energy of 

 the contraction for a short time. This is followed by an in- 

 crease in the irritability, as shown by a gradual increase in the 

 height of the curve until a certain maximum is reached and 

 maintained (Fig. 26). These so-called staircase contractions have 

 been observed in the muscles of both cold-blooded and warm- 

 blooded animals. In time, however, as the muscle becomes 

 fatigued the effects of repeated contractions manifest themselves 

 in a lengthening of the latent period, a diminution in the 

 rapidity and extent of the contraction, and an increase in the 



FIG. 25. SHOWING THE EF- 

 FECTS OF INCREASING 

 STRENGTH OF STIMULUS. 



FIG. 



26. SHOWING STAIRCASE CON- 

 TRACTIONS. 



time of relaxation. If the intervals between ^successive stimu- 

 lations be not sufficient for the muscle to recover itself, the same 

 phenomena arise, though more quickly. 



Temperature. The temperature at which all phases of the con- 

 traction process, as represented by the myogram, attain their 

 physiologic maximum value is about 30 C. If the temperature 

 of the muscle falls to 20 C. there is a corresponding decline in 

 activity, as shown by an increase of the latent period, a decrease 

 in the height of curve, i. e., in the shortening of the muscle, 

 an increase both in the contraction and relaxation periods. As 

 the temperature approaches o C. the height of the curve again 

 suddenly increases, indicating, for some unknown reason, an 

 increase in the irritability. This is, however, scarcely a physio- 

 logic condition. At a temperature of 40 C. to 50 C. the muscle 

 suddenly contracts and passes into the condition of heat rigor. 

 The proteid constituents of the muscle are coagulated and the 

 irritability destroyed (Fig. 27). 



