53 2 MUSCLE-NERVE PHYSIOLOGY 



salt solution, the salt solution apparently brings out the automatic rhythm 

 by an increase in its irritability. Portions of the auricle and of the sinus, 

 especially the latter, are more highly rhythmic than portions of the ventricle, 

 due to a specific difference in the muscle cells themselves and not to the 

 nervous mechanism. 



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

 Circulation. 



15. Involuntary Muscle. Strips of smooth or involuntary muscle, 

 cut from the stomach of a frog or terrapin or from the intestine of a frog, 

 may be used to show the physiology of this character of tissue. Mount a 

 strip in the moist chamber, or in the apparatus shown in figure 356, using 



FIG. 356a. 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. 



* 



care not to load it too heavily; the weight of the ordinary muscle lever may 

 produce too much tension. Stimulate the muscle for one or two seconds 

 with interrupted induction currents of moderate strength. Contractions will 

 follow, usually developing very, very slowly as compared w r ith striated 

 muscle, and lasting through many seconds, from thirty to one hundred 

 seconds. By using very strong inductions occasionally a contraction may 

 be secured with a single stimulus, but single-induction currents as a rule do 

 not produce effective stimuli for smooth muscle, which requires a more 

 slowly developed stimulus. 



If the stomach muscle of the frog be used and it be handled with extreme 

 care, it may happen that automatic contractions will develop in the muscle 

 in the moist chamber. If so, these contractions will be found to be slow and 

 of varying amplitude. The terrapin's stomach muscle will ordinarily not 

 show automatic contractions, but by increasing the temperature to about 



