THE PROPERTIES OF THE HEART MUSCLE 



173 



Considering the automatically contracting muscle, the point in which the 

 automatic contraction is released, i.e., begins, is the point of maximal irri- 

 tability. It is the moment when the irritability is so great that the muscular 

 equilibrium is no longer stable, and the physiological contraction results. 



FiG.^72. Automatic C9ntractions of a Strip of Ventricular Muscle from the Apex of the 

 Terrapin's Heart contracting in 0.7 percent Sodium Chloride; from -j- to + 0.03 per cent Potassium 

 Chloride is added to the Sodium Chloride. The rhythm is recovered very slowly when the muscle 

 isreturnedto o.y-per-cent sodium chloride. Time in minutes (upper) and seconds (lower stroke). 

 (New figure by Watkins and Elliott.) 



The irritability of heart muscle is very sharply influenced by its condition 

 of nutrition, especially by the inorganic salts present in the blood and lymph, 

 see page 1 78. The salt content of the blood comprises about 0.7 per cent sodium 



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. (New figure by L. Frazier.) 



chloride, 0.03 per cent potassium chloride, and 0.025 to 0.03 per cent cal- 

 cium (phosphate probably), as well as traces of other metal bases. The 

 heart muscle has been shown by numerous investigators to be delicately 



