"If two liquids communicate with one another in a capillary tube, or in a porous or parenchymatous structure, and have for that tube or structure different chemical affinities, movements will ensue; that liquid having the most energetic affinity will move with the greatest velocity, and may even drive the other liquid entirely before it."

Starting from this point, he endeavors to show, that the same forces which cause the circulation of sap in the vegetable world are brought into action in the circulation of blood in the human system; that in both cases the principle is the same.

The arterial blood charged with oxygen, as soon as it reaches its destination in the minute capillary vessels, begins to carry on its process of oxydation, attacking in a measured way the various tissues through which it is flowing, burning out their effete carbonaceous matter and in the change of tissue which takes place, causing, as we have already shown, the evolution of heat. While this change is going on in the tissue the arterial blood also undergoes a change, in giving up its oxygen and gaining in exchange the result of the combustion. From crimson, it becomes dark; from arterial it changes to venous blood. On the one side of the capillary tubes, we have of course arterial, on the other venous blood. The arterial blood, bearing its oxygen, ready to burn , out any carbon or hydrogen in its way, substances of which the tube or structure is composed, possesses an intense affinity for those structures, which is at last exhibited by their destruction. The arterial blood has therefore an intense affinity for any of the structures with which it is brought in contact, but after its oxygen is exhausted and the arterial is changed into venous blood, this affinity no longer exists.

Referring back to the principle already laid down, what is the phenomenon which that principle predicts as arising under these circumstances? Simply, that arterial blood will drive the venous blood before it, and drive it with an inexpressible force. From this train of reasoning Dr. Draper concludes that the true cause of the systemic circulation is the oxygenixed action of the arterial blood. In the systemic circulation, upon these principles, the flow must be from the artery to the vein.

In pulmonary circulation we have venous blood presenting itself in the lungs to atmospheric air. The venous blood has an intense affinity for the oxygen in the air, and the arterial, of course none. Movement therefore 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 with great force to the heart. The pulmonary circulation is therefore due to the oxydation of the venous blood.

Those who insist that the circulation is owing to the forcing and suction power of the heart, and to that alone are referred to plants wholly destitute of a heart and yet their juices circulate. There are multitudes of animals in the same predicament. In insects no such central organ appears. In fishes the systemic circulation is carried on without a heart, and in cold-blooded animals movements in the capillaries take place after the heart is cut out When we inquire into the condition of the circulation in the earliest periods of existence, we find that the vessels themselves are the first to appear, and the heart is subsequently developed.

What then is the true office of the heart? The systemic circulation originates in the deoxydation of the blood, the pulmonary, in its oxydation. There is no necessary connexion between the chemical changes taking place in the lungs and those in the system. Both are determined by their own proper causes.

If therefore on either of these points of change an excess of action takes place, the result must be a disturbance of the equilibrium of the whole circulation. At some central point therefore, the current, going to the respiratory machine, and that going to the system must be intercepted - and intercepted by an apparatus which could hold both in check and time the movements of one to those of the other. The heart then by periodic muscular contractions serves to adjust the flowing currents to one another, and prevent engorgements or deficiencies in any part of the route.

How do these chemical principles apply when the system becomes diseased? Whenever the admission of oxygen into the lungs is stopped, the circulation through them ceases. What is the cause of that asphyxiated condition? Why does the blood cease to flow. The chemical theory of the circulation of the blood through the lungs points to the oxydation of that blood as the cause of its movement. It is the pressure of the de-oxydized upon the oxydized blood that drives the latter along the pulmonary veins to the heart. But should anything intervene to prevent that oxydation taking place, no pressure can arise, and therefore no movement ensue; the conditions for asphyxia are all present; conditions which are removed by the readmission of oxygen into the lungs.