The impure blood from the upper part of the fœtal body, which is brought down by the superior vena cava, also enters the right auricle, but does not pass from thence through the foramen, like that from the inferior vena cava. By a peculiar arrangement this blood is made to pass down into the right ventricle, and from thence along the pulmonary artery, the same as in the adult state. Only a very small portion, however, passes into the lungs, the great part being taken along a tube called the _ductus arteriosus_ into the great artery called the aorta, where it begins to turn down to the lower part of the body. In consequence of this, the arterial blood going down to the lower part of the body, is mixed with this portion of impure, venous blood, brought by the ductus arteriosus from the superior vena cava; while that going to the head, and upper part of the body remains pure. And this is the reason why the lower part is always so much smaller than the upper part, previous to birth; it receives less pure nourishment. The head and chest appear, at an early period, almost as large as the rest of the body. This circumstance also explains why, in the great majority of cases, the _right_ arm is preferred to the _left_, and has more real power. The place where the ductus arteriosus pours the impure blood into the aorta, is almost immediately opposite to where the artery is given off which feeds the left arm. In consequence of which, in most cases, a small portion of this impure blood becomes mixed with the arterial blood, and the left arm is, therefore, in the same situation as the lower limbs, and like them is comparatively imperfectly developed. The right arm is not liable to any such deprivation. In some cases the insertion of the ductus arteriosus is lower down, so that no such mixture occurs. Both arms are then equal, and this accounts for the fact that in some persons there appears to be no difference. In some cases, no doubt, early habit, or imitation, may overcome this natural inferiority, and even give the preference to the left arm; but such instances are rare; the general rule is the contrary, and for the reason stated. The ductus arteriosus closes up about the same time as the foramen ovale. The two veins which convey the impure blood back to the mother, to be purified, originate from the iliac artery, in the pelvis. They pass up the sides of the bladder towards the navel, enter the sheath of the cord, and so reach the placenta. These vessels are obliterated about the third or fourth day after birth, and remain afterwards in the form of a fibrous cord.