In the first periods of the fœtus, the layers and arterial fibres are indistinct; we should say that the coat of the artery is homogeneous. But it has however much more consistence than most of the surrounding textures; this consistence corresponds with that of the heart. Destined to distribute every where the nutritive matter, the arteries ought necessarily to precede the other organs in their nutrition. This early growth, always concomitant with that of the heart, would alone prove that the arteries are made to develop themselves, and that the heart does not hollow them out, as Haller has said, in the interior of our organs by the force of its impulse. Besides this mechanical manner of considering their formation is evidently contrary to the known laws of the animal economy. II. _State of the Vascular System with Red Blood during growth._ At the moment of birth, two great revolutions take place in the system with red blood; 1st, a mechanical one, if it may be so said, in the phenomena of the course of the blood; 2d, a chemical one, in the nature of this fluid. The mechanical revolution depends upon the entire cessation of the passage of the blood through the foramen ovale, the ductus arteriosus, the umbilical arteries and veins. The chemical revolution depends upon the formation of red blood. I will now examine this last. The fœtus finds at birth that all surrounding objects are the causes of great excitement; the cutaneous surface, all the origins of the mucous, are strongly stimulated. The sensations they experience are even painful, because the difference is very great between the waters of the amnios and the bodies with which the fœtus comes in contact at birth, and every abrupt change in the sensations is painful. Habit soon familiarizes this sensation; but it is not less real at birth, and it may be said that this moment is as painful for the infant as the mother. Now, as every lively sensation is generally accompanied with great motions a general agitation succeeds the impression that the fœtus perceives from without; all the muscles move, the intercostals and the diaphragm like the others. The air which already filled the mouth and wind-pipe, then enters the lungs, and there colours the blood red, then it is alternately expired and inspired until death. The first inspiration then is, in this point of view, a phenomenon analogous to all the motions that the change of external excitement suddenly produces at birth in the voluntary muscles of the fœtus.