1870.] 



four Orifices of the Heart. 



277 



the recumbent posture, the flow of blood through the heart is entirely and 

 solely under the control of the heart itself (some allowance being made for 

 the effects of the respiratory movements which " act on the whole advan- 

 tageously to the circulation "), the right being filled by the contractile 

 energy of the left side of the organ. In our waking moments, however, 

 during exertion, every movement of the body tends to force the blood in 

 the veins in an onward course towards the right chambers of the heart, 

 which would become gorged from over-distension did not the healthy right 

 ventricle assume corresponding energy and force and expel the blood with 

 increased rapidity into the capillaries of the lungs. An increase in the 

 number and depth of the respiratory movements ensues, accelerating the 

 passage of the blood through the lungs to the left side of the heart, which, 

 by an instinctively increased reaction upon its contents, propels the blood 

 forcibly into the systemic circulation. The so-called vital capillary force 

 or interaction between blood and tissue may assist in forwarding the current, 

 but its amount is evidently excessively small in comparison with the enor- 

 mous contractile energy of the two ventricles. Violent and sudden exertion 

 may for a short time disturb the balance between the two hearts (the cavse 

 and right auricle in one side, and the pulmonary vessels and left auricle in 

 the other side being, for a time, the safety reservoirs or receptacula of the 

 blood waiting to be forwarded) ; but with bodily rest equilibrium becomes 

 rapidly reestablished, and equal volumes of blood are again poured forth 

 in equal and the same times from the two ventricles of the heart. 



Returning from this digression to the immediate subject of this paper, 

 we have to consider the cause of the differences in the areas of the four 

 principal orifices of the heart. 



The right and left sides of that organ are, to all intents and purposes, 

 two distinct and perfect hearts, discharging individually their own proper 

 functions, but associated in one common interest by certain bands of mus- 

 cular fibres and intercommunicating nerve-ganglia. Now if these two 

 hearts had exactly equal tasks to perform and were simply designed to pro- 

 pel the contents of their ventricles to equal distances and with equal velo- 

 cities, if, in a word, they had been intended to overcome equal obstacles 

 in the pulmonic and systemic circulations respectively, their walls would 

 have been undoubtedly constructed of equal thickness, and the corresponding 

 orifices of the two sides would have been of equal areas, the tricuspid being 

 equal to the mitral and the pulmonic to the aortic aperture. But as the 

 left ventricle has to propel the blood to far greater distances, and to over- 

 come obstacles much greater than those found in the pulmonic circulation, 

 the velocity and force of the stream sent from the left must be evidently 

 greater than the velocity and force of the blood thrown out by the right 

 ventricle. To secure this result, I need scarcely say that the left is ren- 

 dered considerably thicker and stronger than the right ventricle by the 

 greater development of its walls ; but here we must bear in mind the car- 

 dinal fact (the key to the entire question) that, whatever be the velocity 



