FffiTAL CIRCULATION. 619 



site, that they may discharge their currents through the same cavity 

 without admixture. 2. That the inferior cava opens almost directly 

 into the left auricle. 3. That by the aid of the Eustachian valve, the 

 current in the inferior cava will be almost entirely excluded from the 

 right ventricle. 



3dly. That the blood which circulates through the arch of the aorta 

 comes directly from the placenta ; and, although mixed with the 

 impure blood of the inferior cava, yet is propelled in so great abund- 

 ance to the head and upper extremities, as to provide for the increased 

 nutrition of those important parts, and prepare them, by their greater 

 size and development, for the functions which they are required to 

 perform at the instant of birth. 



4thly. That the blood circulating in the descending aorta is very 

 impure, being obtained principally from the returning current in the 

 superior cava ; a small quantity only being derived from the left 

 ventricle. Yet is it from this impure blood that the nutrition of the 

 lower extremities is provided. Hence we are not surprised at their 

 insignificant development at birth; while we admire the providence 

 of nature, that directs the nutrient current in abundance to the organs 

 of sense, of prehension, and of deglutition, so necessary even at the 

 instant of birth to the safety and welfare of the creature. 



After birth, the foramen ovale becomes gradually closed by a mem- 

 branous layer, which is developed from the margins of the opening 

 from below upwards, and completely separates the two auricles. The 

 situation of the foramen is seen in the adult heart, upon the septum 

 auricularum, and is called the fossa ovalis; the projecting margin of 

 the opening is the annuls ovalis. 



As soon as the lungs have become inflated by the first act of 

 inspiration, the blood of the pulmonary artery rushes through its 

 right and left branches into the lungs, to be returned to the left 

 auricle by the pulmonary veins. Thus the pulmonary circulation is 

 established. Then the ductus anteriosus contracts, and degenerates 

 into an impervious fibrous cord, serving in after life merely as a bond 

 of union between the left pulmonary artery and the concavity of the 

 arch of the aorta. 



The current through the umbilical cord being arrested, the umbilical 

 arteries likewise contract and become impervious, and degenerate into 

 the umbilical ligaments of the bladder. 



The umbilical vein and ductus venosus, also deprived of their cir- 

 culating current, become reduced to fibrous cords, the former being 

 the round ligament of the liver, and the latter a fibrous band which 

 may be traced along the fissure for the ductus venosus to the inferior 

 vena cava. 



NERVOUS SYSTEM. The brain is very soft, almost pulpy, and has 

 a reddish tint throughout ; its weight at birth, relatively to the entire 

 body, is as one to six, and the difference between the white and grey 

 substance is not well marked. The nerves are firm and well 

 developed. 



