192 



THE CIRCULATION OF THE BLOOD 



alternate 

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systolic and diastolic movements for a varying time. Thus we see 

 power of rhythmic contraction depends neither upon connection 

 with the central nervous system nor yet upon the 

 stimulation produced by the presence of blood 

 within its chambers. Whether or not rhythmicity 

 is a property of heart muscle, as such, was con- 

 clusively settled by Gaskell and by numerous 

 later investigators by a very simple experimental 

 procedure. Gaskell cut thin strips of the apex 

 of the ventricle of the terrapin, which is free from 

 the nerve cells, at least nerve ganglia, and found 

 that they contracted rhythmically for hours. 

 This experiment has become a classic one for 

 the study of the cardiac muscular tissue. Strips 

 of cardiac muscle cut from the auricle and from 

 the contractile walls of the venae cavse, or sinus 

 venosus, of the terrapin also contract rhythmically. 

 If the strips of muscle are kept moist with the 

 same blood or serum, then the rhythm of the sinus 

 is greater than that of the auricle, and that of the 

 auricle greater than that of the ventricle, a differ- 

 ence that is based on a physiological differentia- 

 tion of the tissue. The sinus muscle is also more 

 delicately responsive to stimuli than is the ven- 

 tricular muscle; i.e., it is more irritable. 



Porter has performed the more difficult exper- 

 iment 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 varia- 

 tions of tone were wave-like and periodic, even 



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