The capillary terminations of the pulmonary artery, which ramify in the mucous membrane, forming the parietes of the air-cells, and which hitherto had been firmly compressed by the collapsed state of the foetal lungs, are suddenly rendered pervious throughout their whole extent. By this means, a vacuum, as it were, is formed in the ramifications of the pulmonary artery; each inspiration is accompanied by a rush of blood from the right ventricle into the newly-inflated structure. The pulmonary artery, at its bifurcation, swells and becomes turgid: the blood is carried off into its numerous ramifications as fast as the right ventricle can supply it; this may be easily understood from the law, in anatomy, viz. that the area of two arteries is greater than that of the trunk from which they bifurcate. From this state of distension, the distance between the pulmonary artery and the aorta is increased; the ductus arteriosus, which has now become empty, is stretched, and thus partially closed; the right auricle, which, but for the foramen ovale, could not have cleared itself of the whole quantity of blood which was poured into it from the vena cava, is now enabled to transmit its entire contents into the right ventricle; the left auricle, which before birth was supplied only by the foramen ovale from the right auricle, is now rapidly filled by the blood brought into it by the four pulmonary veins;--the equilibrium between the two auricles becomes altered;--the right, which hitherto had been somewhat gorged with blood, is now able to clear itself with facility; whereas, the left, which was but partially supplied, is now distended with a much greater quantity: there is now rather a disposition for the blood to regurgitate from the left to the right auricle; this, however, is prevented by the semilunar fold of the foramen ovale, which now acts as a valve, and generally becomes firmly attached to the septum. The obliteration of the canalis venosus at the posterior margin of the liver, and of the umbilical vein at the anterior edge, may, we think, be explained by the changes which necessarily follow the inflation of the lungs: the diaphragm, when it contracts, pulls down the liver into its natural situation; the distance, therefore, between the liver and the heart is increased, and the canalis venosus is consequently stretched, and considerably pressed upon, and precisely the same results follow with the umbilical vein. PART II NATURAL PREGNANCY AND ITS DEVIATIONS. CHAPTER I. SIGNS OF PREGNANCY.