172 



THE CIRCULATION OF THE BLOOD 



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auricle, and that of the auricle greater than that of the ventricle, a difference 

 that is based on a physiological differentiation of the tissue. The sinus 

 muscle is also more delicately responsive to stim- 

 uli than is the ventricular muscle, i.e., it is more 

 irritable. 



Porter has performed the more difficult ex- 

 periment of isolating a small disc of muscle from 

 the ventricle of the dog, leaving only the delicate 

 nutrient artery through which the muscle was 

 fed with defibrinated blood. This isolated small 

 piece of ventricle contracted vigorously for many 

 minutes. We may conclude, then, that the 

 mammalian heart muscle is also automatically 

 rhythmic. 



Tonicity. Cardiac muscle is characterized by 

 its maintaining a constant degree of partial con- 

 traction described as muscle tone, or tonicity. 

 This property is possessed by all parts of the 

 heart. In the auricle, however, and especially in 

 the muscular walls of the sinus and veins, there is 

 considerable variation in tonicity. Botazzi showed 

 that in the auricle of the toad the variations of 

 tone were wave-like and periodic, even though the 

 auricle were contracting with its ordinary funda- 

 mental rhythm. Howell has published numerous 

 experiments showing tone waves in auricular and 

 sinus muscle of the terrapin, in which muscle there 

 may or may not be occurring at the same time 

 the ordinary fundamental rhythmic contractions, 

 figure 170. 



Irritability of Heart Muscle. Mention was 

 made above of the difference in irritability of heart 

 muscle chosen from different parts of the heart. 

 The irritability of the muscle of each part also 

 varies during the different stages of the contrac- 

 tion. When a contraction occurs, experiment 

 shows that the muscle is not irritable to a special 

 stimulus applied at any time from the beginning 

 of the contraction until the summit of the con- 

 traction is reached. This is called the refractory 

 period. From the summit, through the relaxation 

 and succeeding pause, the irritability rapidly in- 

 creases until the beginning of the next contraction. 



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