The circulation of the black blood is performed in a manner the reverse of the preceding. It begins in the general capillary system; it is in this system, that its blood takes the peculiar character that distinguishes it from the preceding; it is here that it is formed, by the subtraction probably of the principles of the air that it acquired by terminating its course in the lungs. From this general capillary system, it enters the veins which transmit it to the right cavities of the heart, which send it by the pulmonary artery, to the capillary system of the lungs. This system is its real termination, as it is the commencement of the circulation of the red blood. A general membrane, every where continued, lines the whole course of the black blood, and forms for it also a general and continuous canal, in which it is constantly carried from all parts to the interior of the lungs. At the exterior of this great canal, nature has placed a loose membrane in the veins, fleshy fibres in the heart, and a peculiar fibrous texture in the pulmonary artery; but, like the preceding canal, it remains always nearly uniform, notwithstanding this difference of organs to which it is united externally. It is by the folds of this membrane in the veins that the valves are formed. It contributes to form all those of the right side of the heart, whose cavities it lines, as the preceding enters into the composition of the valves of the left side, which borrows from it the membrane that lines it. _Difference of the two circulations._ From the general idea that I have given of the two circulations, it is evident that they are perfectly independent of each other, except at their origin and termination, where the red and black blood are alternately transformed into each other, and communicate for this purpose by the capillary vessels. In their whole course they are entirely separate. Though the two portions of the heart are united in one single organ, they may however be considered as uniformly independent in their action. There are truly two hearts, a right and a left. Both would be able perhaps to perform their functions as well if they were separate, as they now do united. When the foramen ovale remains open after birth, I have proved elsewhere that such is the arrangement of the two folds between which it is found, that the black blood cannot communicate with the red, and that the two hearts should equally be considered independent, at least as it respects the course of the blood. This entire separation of the two circulations is one of their most striking characters; it alone proves how much preferable the point of view in which I have presented the circulation, is to that which divides it into great and small, which are evidently confounded and identified.