568 



MUSCLE-NERVE PHYSIOLOGY 



ing the descending current or on opening the ascending current. On 

 reversing the current the opening stimulus will not be effective. 



20. Cardiac Muscle. Cardiac muscle differs from voluntary in that 

 the contractions occur rhythmically and automatically. This is shown by 

 the isolated frog's heart, which continues to contract when bathed with 

 blood or salt solution, often for hours. This isolated heart, however, has a 

 complicated local nervous mechanism. The apex of the ventricle of the 

 terrapin's heart is practically free from nerve ganglia and is used to demon- 

 strate the characteristics of pure cardiac muscle. 



a. Cut a strip off the apex of the terrapin's ventricle, as shown in figure 

 214, and mount it by means of light silk-thread ligatures tied around the two 

 ends of a strip and attached to the apparatus shown in figure 356. The loop 

 of the lower thread should be i cm. long, of the upper thread long enough to 



FIG. 356. Arrangement of Apparatus for Studying the Contractions of the Strip of the 



Apex of the Ventricle. 



reach to the lever above. When such ventricular strips are immersed in 

 ordinary 0.7 per cent, sodium chloride they will begin contractions in a few 

 minutes, twenty minutes or so. The contractions are regular in rate and will 

 continue through two or three hours, gradually becoming smaller and smaller 

 until a standstill is reached. If the strip is immersed in its own serum it will 

 give only occasional contractions, but it remains 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. 

 b. Portions of the auricle and of the sinus, especially the latter, are more 

 highly rythhmic than portions of the ventricle, due to a specific difference in 

 the muscle cells themselves rather than to the nervous mechanism contained. 

 Prepare a strip from the auricle and compare it with the ventricle. 



