THE PROPERTIES OF THE HEART MUSCLE 193 



though the auricle were contracting with its ordinary fundamental rhythm. 

 Howell has published numerous experiments showing tone waves in 

 auricular and sinus muscle of the terrapin, in which muscle there may 



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. Postassium Chloride is added to the Sodium Chloride. The rhythm is recovered 

 very slowly when the muscle is returned to o . 7 per cent, sodium chloride. Time in 

 minutes (upper) and seconds (lower stroke). (New figure by Watkins and Elliott.) 



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 



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. (Xew figure by L. Frazier.) 



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 contraction. Experiment shows that the muscle is not irritable to a 

 13 



