The blood is then evidently divided in the first months after conception into two circulations, which cross, if it may be so said, in the form of the figure 8, as has been remarked by Sabatier; it is carried in each, from one assemblage of capillaries to another of the same vessels. Only instead of moving between the pulmonary capillary system and the general one, as in the adult, it moves between the superior and inferior part of this last system; we may then say in this point of view that the inferior and superior parts of the body are in opposition in the fœtus, as the lungs in the adult are in opposition to the rest of the body. This complete opposition on the part of the circulation, between the upper and lower part of the body, in the first months of the fœtus, is probably the origin of the difference that takes place afterwards between these parts. All physicians have observed this difference in diseases. If the median line frequently separates the affections of the right side from those of the left, the diaphragm seems often to be the boundary of many diseases. Who does not know, that scorbutic affections appear particularly below, that serous infiltrations are most frequent there, and that ulcers are infinitely more common in the inferior extremities, and that on the other hand, most cutaneous eruptions take place in the superior parts, &c.? Bordeu, who has said much of the division of the body into superior and inferior parts, considered one pulse as the precursor of evacuations from above, and another as that of those from below, he has however without doubt exaggerated this opposition of the two parts of the body; still it really exists, and I think that it is very probable, that the manner of the circulation of the fœtus is the primary source of it. After the first months, things begin to change. The quantity of blood passing by the pulmonary artery was at first scarcely any thing, because the dilatation of the ductus arteriosus was so great, that it turned almost the whole of it into the descending aorta. This canal gradually contracting, the pulmonary arteries dilate, and then more blood goes through the lungs, and is brought by the pulmonary veins to the left auricle, which transmits it to the left ventricle, which sends it to the arch of the aorta; then the mechanism of the circulation described above begins to change, and approximate that of the infant, as we shall see.