266 THE CIRCULATION OF THE BLOOD 



In pure serum or blood the rhythmicity is entirely absent though it is 

 significant that the few sporadic contractions that do occur are large. 



This preparation makes possible many instructive experiments tending 

 to show fundamental properties of cardiac muscle. The preparation 

 contains no nervous mechanism except terminal fibers, and its behavior 

 may be safely attributed to the muscle substance itself. 



Try the following experiments: i. Submit the strip to saline solutions 

 of different temperatures, varying through steps of 10 degrees from o C. 

 to 30 C. 2. Try the effect of the different ingredients in Ringer's solution 

 using physiological saline as the standard normal for the ventricle, see Exp. 

 6, b, c, etc. 



a. Combine potassium chloride with saline, figure 172; 



b. Calcium chloride with saline, figure 173; 



c. Potassium and calcium chloride and saline; 



d. Locke's solution; 



e. Solution of blood diluted with saline; 



/. Solution of milk with saline in the proportion of one part milk to 

 four of saline. 



Cut and mount strips from the auricle and from the sinus, letting the 

 latter extend out on to the vena cava. In these last preparations care 

 must be taken to balance the lever, as a slight overtension paralyzes the 

 muscle. 



Immerse these strips in pure serum, or Ringer but not physiological 

 saline, and compare their behavior with that of the ventricle in pure serum. 

 The sinus and usually the auricle will be found rhythmic in serum, while 

 the ventricle, if it contracts at all, will contract at irregular periods. 

 Often there is a distinct progressive decrease in the rhythm, the sinus 

 having the same rhythm as the whole heart, the auricle a considerably 

 slower rhythm, and the ventricle slow and aperiodic. The sinus prepa- 

 ration will show beside the fundamental rhythm a characteristic slow 

 contraction and relaxation, which has been described as tone, figure 170. 

 Repeat Exp. 6, a, b } c, etc., on the auricle and sinus. 



ii. Influence of the Cardiac Nerves on the Frog's Heart. Care- 

 fully pith a frog with the minimal loss of the blood of the animal. Expose 

 the heart as previously described, make a cut through the manubrium, 

 continue it through the skin and muscles at the angle of the jaw, thus 

 exposing the vagus nerve. The vagus runs along the edge of a delicate 

 muscle diagonally downward and backward toward the heart. The 

 glosso-pharyngeal is just in front of the vagus and the hypoglossal just 

 behind it. The latter runs parallel with the vagus near its origin, but 

 lower down turns across the vagus and runs to its distribution in the 

 tongue muscles. These two nerves serve to aid the student in the identi- 

 fication of the vagus, see figure 217. It is usually better to cut the 



