DUGALD E. S. BROWN 



105 



to a level just equalling the pressure inhibition of tension development 

 by the activated unit. 



Isometric Twitch of Skeletal Muscle. The information regarding the 

 sequence of events in the isometric twitch of skeletal muscle is so extensive 

 that the preceding results on cardiac muscle which are not so fully known 

 may be considered to better advantage in this context. In the beginning it 

 may be emphasized that the response of cardiac muscle to abrupt changes in 

 pressure confirms earlier results on the retractor penis of the turtle and on 

 the sartorius of the frog {R. pipiens) (43 ». In the retractor penis, appli- 



FiG. 13. Myograms illustrating the augmentation in tension produced by an abrupt 

 compression to 272 atmospheres during the contraction cycle. Retractor penis muscle 

 of the turtle. Temperature 20°C. Myogram A recorded at atmospheric pressure. Myo- 

 gram B recorded at 272 atmospheres, tlie pressure being applied 15 seconds before 

 stimulation. In remaining records, the moment of compression is indicated by the 

 upper arrow, P, while the moment of stimulation is indicated by the lower arrow, 

 S. Time scale equals 0.04 second per division. 



cation of a pressure of 4,000 psi prior to stimulation and maintained 

 throughout the contraction causes a considerably larger twitch which 

 is also somewhat prolonged (fig. 13T, B; fig. 14). In contrast, an abrupt 

 compression applied when one-tenth of the tension has developed at 

 atmospheric pressure and the pressure maintained thereafter produces 

 a twitch not particularly different from the control (fig. 13F, 14). The 

 converse of this experiment, done at 3°C and employing an abrupt de- 

 compression, shows that compression for only the first one-tenth of the 

 contraction results in a twitch tension 95% of that developed under a 

 sustained pressure (44). From this it was concluded that in this brief 

 period pressure sets up a condition which determines the duration of the 



