INVOLUNTARY MUSCLE 501 



mains irritable and capable of contracting at any moment. If changed to 

 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 mus- 

 cle, 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 354 A, using 



FIG. 355. 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 with 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 



