As the ductus venosus and hypogastric arteries terminate in blind ends and become useless as soon as the umbilical cord is cut, they soon begin to atrophy and are obliterated within a few days after birth. This means that less blood is poured into the right auricle, which naturally results in relatively less tension in the right heart and an increased pressure in the left, which tends to close the foramen ovale. The foramen ovale does not entirely disappear at once, however, but closes gradually, sometimes remaining open for months. Occasionally it remains open permanently, and though some people have gone through life comfortably with a patent foramen ovale, its ultimate failure to close usually results in serious circulatory trouble. This is also true of the ductus arteriosus, which sometimes, but not often, fails to close. The rule is that as the lungs expand and an increased amount of blood is carried to them for aëration, the ductus arteriosus deflects a steadily diminishing stream from the right ventricle to the arch of the aorta. Thus it gradually ceases functioning in most cases and disappears in the course of a few weeks. The abandoned vessels may degenerate and disappear in time or they may persist in the form of small fibrous cords. (Fig. 29.) Although the circulatory system shows the most elaborate adjustments to the protection afforded by intra-uterine life, there are also other adaptations made by the fetal organism. The baby acquires about 90 per cent of its weight during the latter half of pregnancy, as well as a steadily increasing proportion of solids and a decrease of fluids in its tissues, for in its early days the embryo consists largely of water. But for all of that, its existence and growth in utero, and the functioning of its heat producing centre require surprisingly little oxygen and nourishment. The amniotic fluid keeps the fetus at an equable temperature, about 1° above that of the mother, and as space within the uterine cavity permits of only limited movement, there is very little combustion for the liberation of heat and energy. =The kidneys= assume functional form at a very early fetal age, probably about the seventh week, and the presence of albumen and urea in the amniotic fluid suggest that small amounts of urine may be voided, particularly during the latter part of pregnancy. =The bowels=, on the other hand, are normally inactive, this is in spite of the fact that the baby evidently obtains fluid, and possibly some nutriment by swallowing amniotic fluid. But a discharge of meconium may be caused by pressure on the cord or by any condition which interferes with the umbilical circulation. For this reason, meconium stained fluid escaping during labor in a head presentation may be taken as an evidence of imminent asphyxiation, due to an interruption of the umbilical circulation.