CARDIAC MUSCLE 569 



c. Repeat with sinus strips using care to compensate the weight of the 

 lever. 



Refer to the experiments on cardiac muscle at the end of the chapter on 

 Circulation. 



21. Involuntary Muscle, Coldblooded Animal. Strips of smooth or 

 involuntary muscle cut from the stomach of the frog will show the physio- 

 logical reactions of this type of muscle. Mount a spiral strip of frog 

 stomach muscle in the Harvard warm chamber holder. Use a wire to 

 connect the muscle with the lever as in figure 356. Take care not to 

 load too heavily, the ordinary muscle lever is often too great a tension. 

 Mount a signal magnet to automatically mark the time of stimulation. 



a. Frog stomach muscle will develop automatic contractions in a few 

 moments. Record through two or three circuits of the drum. The speed 

 should be i mm. per second. 



b. If automatic contractions do not develop stimulate the muscle 

 by interrupted currents of varying strength. After a variable latent 

 period contractions will develop. If the automatic contractions are occur- 

 ring, stimulation will increase the amount and sometimes the frequency 

 of the contractions. 



FIG. 3560. Figure Showing the Type of Contraction of a Strip of Muscle from the 

 Stomach of a Frog. The muscle was stimulated with an interrupted current during the 

 time indicated by the signal tracing, immediately below the time tracing. Time in seconds. 



c. A strip of terrapin stomach muscle will not develop automatic 

 rhythm but will respond to comparatively strong stimulation. Increasing 

 the temperature to 30 will often initiate rhythm. 



d. Smooth muscle, like voluntary muscle, responds to variation in 

 temperature, fatigue, strength of stimulus, etc. It is more sensitive to 

 injury. It shows a contraction amplitude proportional to the stimulus. 



