1870.] 



four Orifices of the Heart, 



281 



the passage of the same quantity of blood through the latter opening. The 

 tricuspid is nearly three times larger than the aortic aperture, and is 

 open for the transmission of the same volume of blood more than double 

 the length of time occupied by the latter opening. Hence the comparative 

 slowness of the incoming tricuspid current. 



These speculations upon the absolute and relative velocities of the 

 currents of blood through the heart are not without practical value, in- 

 asmuch as they have a direct bearing upon the question of the amount 

 of pressure exerted by that fluid in er^ch chamber of the organ, and are 

 links in the chain of reasoning respecting the comparative areas of the 

 four orifices. The first-recorded experiments to determine this pressure 

 were made by Dr. Stephen Hales, F.R.S., and were published by him in 

 his ' Statical Essays ' in 1732. Thus, when tubes were fixed into the cru- 

 ral artery and jugular vein of different animals, the heights to which the 

 blood rose were found to be as follows : — 



Artery. Vein. 



Horse 114 inches. 1 2 inches. 



Sheep 77j o|- 



Dog 48 4 



These experiments were, of course, rather roughly made and without 

 modern appliances ; but they serve to show that the pressure of the blood 

 in the jugular vein is only one-ninth to one-fourteenth of the pressure observed 

 in the arterial side of the circulation. Valentin, by means of the hsemady- 

 namometer, estimated the pressure in the jugular vein to be one-tenth to one- 

 twelfth of the pressure in the carotid artery, and "in the upper part of the 

 inferior vena cava could scarcely detect the existence of any pressure, 

 nearly the whole force from the heart having been apparently consumed 

 during the passage of the blood through the capillaries" (Kirkes and 

 Paget). 



It is thus sufficiently clear, experimentally, that the velocity and mo- 

 mentum of the blood which enters the right auricle and finds its way into 

 the right ventricle must be very small in comparison with the rapidity and 

 momentum of the current issuing from the left ventricle ; and we can 

 therefore, from this fact, understand that the tricuspid is constructed of 

 much greater area than the aortic opening, in order that its much larger 

 orifice may compensate for the comparatively sluggish stream which it has 

 to transmit. It is evident enough why the blood which has returned to 

 the right heart possesses so small an amount of velocity and momentum. 

 In its passage through the systemic circulation it has encountered and 

 overcome an amount of obstruction which, by the time it has arrived in 

 the right auricle, has deprived it of the greater portion of the velocity and 

 momentum which it had derived from the contractile energy of the left 

 ventricle, assisted, as that power has been, by the muscular pressure on 

 the veins of the body. The columns of blood from the superior and infe- 



