266 



Dr. Herbert Davics on the Areas of the. [Mar. 17, 



tricuspid, pulmonic, mitral, and aortic orifices — the four principal openings 

 in the heart. 



Although to ordinary observation these orifices appear to exhibit no 

 mutual relationship of size, there can be no doubt that an instrument so 

 accurate in the adaptation of its valvular apparatus, and so exact in the 

 working of its different parts, must reveal on close examination the exist- 

 ence of laws which not only determine the force required to be impressed 

 upon the blood traversing its chambers, but also the relative sizes of these 

 apertures to one another. 



The facts and inferences which I shall adduce on this subject will tend, 

 I believe, to throw some light upon the mechanism of the heart in its 

 healthy state, and will explain also some points of practical interest in the 

 pathology of that organ; 



To M. Bizot in France, and Drs. Peacock and Reid in this country, are 

 we mainly indebted for the most careful and trustworthy measurements of 

 the circumferences of the orifices. Their measurements have been re- 

 corded to the minuteness of the thousandth part of an inch, and yet it will, 

 I believe, be readily admitted that the results in the form given by these 

 distinguished observers help us but little in obtaining any definite idea of 

 the mutual relationship of the areas of the orifices, and are destitute of any 

 practical value in our study of the mechanism of the heart itself. 



Had the observations been pushed further, or, rather, had the figures 

 been worked out into some distinct and definite shape, these observers 

 could not have failed to discover an interesting and important law pre- 

 siding over the areas of these orifices, and they would thus have been 

 enabled to utilize a multitude of measurements which had been obtained 

 by considerable labour and patient research. 



Taking the measurements given by Dr. Peacock in the Croonian Lec- 

 tures for 1865, we find the mean circumferences of the four orifices, ex- 

 pressed in English inches, to be as follows : — 



Males. Females. 



Tricuspid 474 4'562 



Pulmonic 3'552 3*493 



Mitral 4 3-99G 



Aortic 3-14 3*019 



I will now place these valuable facts into another shape by calculating 

 from these measurements of circumference the areas of the respective 

 openings. 



The circumference of a circular opening being known, its radius is de- 

 termined from the formula 



circumference = 2irr i 

 where 7r=3'1415 ; and the radius being thus determined, the area of the 

 opening is calculated from the formula 



area = 7r?' 2 . 



