CIRCULATION OF THE BLOOD. 



123 



Atmospheric Pressure. The frequency of the pulse increases in a 

 corresponding ratio with the elevation above the sea. 



Temperature. The rapidity and force of the heart's contractions are 

 largely influenced by variations of temperature. The frog's heart, when 

 excised, ceases to beat if the temperature be reduced to 32 F. (0 C.). 

 When heat is gradually applied to it, both the speed and force of the 

 heart's contractions increase till they reach a maximum. If the tem- 

 perature 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 temperature of 

 the air was maintained at 105 F. (40.5 C.). At 113 114 F. (45 C.), 

 the rabbit's heart ceases to beat. 



Relative Frequency of the Pulse to that of Respiration. 



In health there is observed a nearly uniform relation between the fre- 

 quency of the pulse and of the respirations; the proportion being, on an 

 average, one respiration to three or four beats of the heart. The same 

 relation is generally maintained in the cases in which the pulse is naturally 

 accelerated, as after food or exercise; but in disease this relation usually 

 ceases. In many affections accompanied with increased frequency of the 

 pulse, the respiration is, indeed, also accelerated, yet the degree of its 

 acceleration may bear no definite proportion to the increased number of 

 the heart's actions: and in many other cases, the pulse becomes more fre- 

 quent without any accompanying increase in the number of respirations; 

 or, the respiration alone may be accelerated, the number of pulsations re- 

 maining stationary, or even falling below the ordinary standard. 



The Force of the Ventricular Systole and Diastole. The 

 force of the left ventricular systole is more than double that exerted by the 

 contraction of the right: this difference in the amount of force exerted 

 by the contraction of the two ventricles, 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 intra- ventricular 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 | of an 

 inch ( 17 to 16 mm.), and in the left ventricle from 2 inches to $ of 

 an inch ( 52 to 20 mm.). Part of this fall in pressure, and possibly 

 the greater part, is to be referred to the influence of respiration; but with- 

 out this the negative pressure of the left ventricle caused by its active 

 dilatation is about | of an inch (23 mm.) of mercury. 



The right ventricle is undoubtedly aided by this suction power of the 



