THE CIRCULATION OF THE BLOOD. 127 



effort of the individual, it has beeu proved that the increased frequency 

 of the pulse in the sitting and standing positions is dependent upon the 

 muscular exertion engaged in maintaining them ; the usual effect of 

 these postures on the pulse being almost entirely prevented when the 

 usually attendant muscular exertion was rendered unnecessary. (Gruy.) 



(7.) Atmospheric Pressure. The frequency of the pulse increases in 

 a corresponding ratio with the elevation above the sea. 



(8. ) 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 32F. (o 

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

 the contractions increase till they reach a maximum. If the tempera- 

 ture 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-L14 F. (45 

 C.), the rabbit's heart ceases to beat. 



Relative Frequency of the Heart's Contractions to the number of Res- 

 pirations. In health there is observed a nearly uniform relation between 

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

 portion being, on an average, one respiration to three or four beats. 

 The same relation is generally maintained in the cases in which the ac- 

 tion of the heart is naturally accelerated, as after food or exercise; but 

 in disease this relation usually ceases. 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 proportion to the increased number of the heart's actions : and 

 in many other cases, the heart's contraction becomes more frequent with- 

 out any accompanying increase in the number of respirations ; or, the 

 respiration alone may be accelerated, the number of pulsations remain- 

 ing stationary, or even falling below the ordinary standard. 



The Force of the Ventricular Action. The force of the left ven- 

 tricular 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 

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

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



