146 CIRCULATION OF THE BLOOD. 



the constituents of the blood ; they take the material they need and their 

 anHity is satisfied ; or secreting cells originate a drain upon the blood, 

 and the moment they have removed from it the substance to be secreted, 

 they have no longer any relation with it. So processes of oxidation, and 

 processes of nutrition, and processes of secretion, all conspire to draw the 

 current onward from the arteries, and to push it out toward the veins ; 

 and though these processes may present themselves in many various as- 

 pects, they are all modifications of the same simple physical principle. 



The blood has now reached the veins, and is forced onward in them by 

 the power that has thus originated in the capillaries. The influence of 

 the heart is here unfelt, the exhausting action of its right auricle is un- 

 appreciable, and, thus pushed onward from the capillaries, it reaches the 

 heart, completing its systemic or greater circulation. This circulation 

 may therefore be said to be due to the high affinity which arterial blood 

 has for the tissues, venous blood having none ; and the action of the heart 

 is confined to the filling of the arterial tubes, and presenting fresh por- 

 tions of blood to the capillaries. 



Arrived at the right auricle, the blood flows continuously into it and 

 the right ventricle for a moment, but the ventricle holding more than the 

 auricle, the latter cavity is fully distended first. At that instant it con- 

 tracts, the valves in the veins shutting, and the blood, driven thus forcibly 

 into the ventricle, distends it to the utmost. The ventricle, in its turn, 

 now contracts, the tricuspid valve shutting, and th*blood issues forth 

 through the pulmonary artery, its valves then closing. At this moment 

 an event occurs which, in these descriptions, is generally overlooked an 

 action analogous to that of the hydraulic ram. On the shutting of the 

 tricuspid, the whole column of venous blood would be brought to a stop 

 if the tubes containing it were unyielding, and a great force would be gen- 

 erated from this stopping of its momentum ; but the auricle is ready to 

 dilate, and into its cavity the blood, which would be otherwise checked, 

 flows. I consider that this safety action of the auricle is one of its prime 

 functions. The rapidity with which the dilatations and contractions are 

 taking place furnish no argument against the occurrence of this action. 

 I have a hydraulic ram, the pulsations of which may be so adjusted as 

 to exceed greatly in frequency those of the heart, and, indeed, to give rise 

 to a low murmuring sound, and yet, under these circumstances, the lat- 

 eral force is so great as to throw a column of water more than forty feet 

 high. If it were not for the dilatability of the auricles and their yield- 

 ing texture, the veins would burst on the shutting of the tricuspid valve. 



The ramifications of the pulmonary artery bring the blood to the cap- 

 illaries of the lungs, but beyond that the influence of the heart is not felt, 

 for now the physical principle heretofore described comes again into ac- 

 tion. The venous blood has a high affinity for the oxygen of the air, an 



