198 THE CIRCULATION OF THE BLOOD 



fashion. We may safely conclude, therefore, that the mammalian heart 

 muscle is also automatically rhythmic. 



Tonicity. Cardiac muscle is characterized by its maintaining a con- 

 stant degree of partial contraction described as muscle tone, or tonicity. 



FIG. 172. Automatic Contractions of a Strip of Ventricular Muscle from the Apex 

 of the Terrapin's Heart contracting in 0.7 per cent. Sodium Chloride; from + to + 0.03 

 per cent. Potassium Chloride is added to the Sodium Chloride. The thythm is recovered 

 very slowly when the muscle is returned to 0.7 per cent, sodium chloride. Time in 

 minutes (upper) and seconds (lower stroke). (Watkins and Elliott.) 



This property is possessed by all parts of the heart. In the auricle, how- 

 ever, and especially in the muscular walls of the sinus and veins, there is 

 considerable variation in tonicity. Botazzi showed that in the auricle of 



FIG. 173. Automatic Contractions of a Strip of Ventricular Muscle from the Apex 

 of the Terrapin's Heart, a, Contracting in 0.7 per cent, sodium chloride; b, when 0.03 

 per cent, calcium chloride solution is added. Time in minutes. (Frazier.) 



the toad the variations of tone were wave-like and periodic, even though 

 the auricle were contracting with its ordinary fundamental rhythm. 

 Howell has published numerous experiments showing tone waves in auri- 

 cular and sinus muscle of the terrapin, in which muscle there may or may 



