204 HANDBOOK OF PHYSIOLOGY. 



C. (32 F.). When heat is gradually applied to it, both the speed 

 and force of the contractions increase till they reach a maximum. If 

 the temperature is still further raised, the beats become irregular and 

 feeble, and the heart at length stands still in a condition of "heat- 

 rigor." Similar effects are produced in warm-blooded animals. In the 

 rabbit, the number of heart-beats is more than doubled when the tem- 

 perature of the air was maintained at 40.o C. (105 F.). At 45 C. (113 

 114 F.), the rabbit's heart ceases to beat. 



In health there is observed a nearly uniform relation between the 

 frequency of the beats of the heart and of the respirations; the propor- 

 tion being, on an average, 1 respiration to 3 or 4 beats. The same rela- 

 tion is generally maintained in the cases in which the action of the heart 

 is naturally accelerated, as after food or exercise; but in disease this 

 relation may cease. In many affections accompanied with increased 

 frequency of the heart's contraction, the respiration is, indeed, also 

 accelerated, yet the degree of its acceleration may bear no definite pro- 

 portion to the increased number of the heart's actions: and in many 

 other cases, the heart's contraction becomes more frequent without any 

 accompanying increase in the number of respirations; or, the respiration 

 alone may be accelerated, the number of pulsations remaining station- 

 ary, or even falling below the ordinary-standard. 



The Force of the Cardiac Action. 



(a.) Ventricular. The force of the left ventricular systole is more 

 than double that exerted by the contraction of the right ventricle: this 

 difference results from the walls of the left ventricle being about twice 

 or three times as thick as those of the right. And the difference is 

 adapted to the greater degree of resistance which the left ventricle has 

 to overcome, compared with that to be overcome by the right: the 

 former having to propel blood through every part of the body, the latter 

 only through the lungs. The actual amount of the intraventricular 

 pressures during systole in the dog has been found to be 2.4 inches (60 

 mm.) of mercury in the right ventricle, and 6 inches (150 mm.) in the 

 left. 



During diastole there is in the right ventricle a negative or suction 

 pressure of about f of an inch ( 17 to 16 mm.), and in the left ven- 

 tricle from 2 inches to i of an inch ( 52 to 20 mm.). Part of this 

 fall in pressure, and possibly the greater part, is to be referred to the in- 

 fluence of respiration; but without this the negative pressure of the left 

 ventricle caused by its active dilatation is about equal to f of an inch 

 (20 mm. )of mercury. 



The right ventricle is undoubtedly aided by this suction power of 

 the left, so that the whole of the work of conducting the pulmonary 



