36 CAUSE OF THE PULMONARY CIRCULATION. 



112. To the mind of a chemist, the relation which exists between arterial and venous 

 blood and the soft solids of the animal body may be very forcibly impressed by these 

 considerations. Regarding the blood itself as a mere vehicle for the introduction of 

 oxygen, and the carrying away of carbonic acid and water, and remembering that the 

 substances of the tissues acted upon are chiefly carbon and hydrogen, our final estimate 

 of the relation of arterial and venous blood respectively to those tissues comes to this 

 the affinity of arterial blood is expressed by the affinity of oxygen for carbon and hy- 

 drogen ; the affinity of venous blood by that of carbonic acid for carbon, and of water 

 for hydrogen. Compared together, therefore, the former is the representative of a 

 highly energetic force, which in the latter is diminished down to zero. 



113. Now what is the phenomena which our general principle (108) predicts, as 

 arising under these circumstances ? Simply this, that the arterial will drive the venous 

 blood before it, and drive it with an inexpressible force. 



114. The oxygenizing action of the arterial blood is, therefore, the true cause of the 

 systemic circulation. 



115. In the systemic circulation, upon these principles, the flow must be from the 

 artery to the vein. 



116. The pulmonary circulation next presents itself. In this, as we have described 

 (106), there is a certain chemical change going on, the consideration of which gives 

 us the cause of the movement. We have seen that the motive force of the systemic 

 circulation arises from the deoxydation of arterial blood (114). How is it with the 

 pulmonary ? We have here venous blood presenting itself on the air-cells, no longer 

 presenting itself to carbonaceous or hydrogenous atoms, such as constitute the soft 

 solids, but presenting itself to atmospheric air, or, more truly, to oxygen gas itself, which, 

 being the more absorbable of the constituents of the air, is taken up and held in solu- 

 tion by the moist walls of the air-cells. Under these circumstances, we see plainly 

 that we are considering a case which is precisely the converse of the former ; in that 

 the arterial blood had an intense affinity for the carbonaceous substances with which 

 it was brought in contact, and the venous none. In this, the venous blood has a cor- 

 responding intense affinity for the oxygen which is dissolved in the tissues with which 

 it is in contact, and the arterial blood has none. Movement again must ensue, but as 

 the conditions of the affinity are reversed, so also is the direction of the motion, for 

 now the venous blood drives the arterial before it, and drives it with an inexpressible 

 force to the heart. 



117. The pulmonary circulation is, therefore, due to the oxydation of the venous 

 blood. 



118. The direction of the pulmonary circulation ought to be from the venous to the 

 arterial side. 



119. Had we, therefore, known nothing of the circulation in the higher order of 

 animals, but been instructed in the chemical relations of the blood to the soft tissues 

 and atmospheric air, we could, upon physical principles, have predicted the existence 

 of that circulation, and shown what its direction in different organs must be. 



120. It may strike those who are not familiar with physiological facts, that in these 



