. A text-book of animal physiology, with introductory chapters on general biology and a full treatment of reproduction ... Physiology, Comparative. THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 121 centers) wMcli preside over respiration, owing to its being roused into action by the lack of its accustomed supply of oxygen, the hitherto solid lungs are expanded; the pulmonary vessels are rendered permeable, hence the blood now takes the path of least resistance along them, as it formerly did through the ductus arteriosus. The latter, from lack of use, atrophies in most instances. The blood, returni
. A text-book of animal physiology, with introductory chapters on general biology and a full treatment of reproduction ... Physiology, Comparative. THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 121 centers) wMcli preside over respiration, owing to its being roused into action by the lack of its accustomed supply of oxygen, the hitherto solid lungs are expanded; the pulmonary vessels are rendered permeable, hence the blood now takes the path of least resistance along them, as it formerly did through the ductus arteriosus. The latter, from lack of use, atrophies in most instances. The blood, returning to the left auricle of the heart from the lungs in increased volume, so raises the pressure in this chamber that the stream that formerly flowed through the foramen ovale from the right auricle is opposed by a force equal to its own, if not greater, and hence passes by an easier route into the right ventricle. The fold that tends to close the foramen ovale grows gradually over the latter, so that it usually ceases to exist in a few days after birth. At birth, ligature of the umbilical cord cuts off the placental circulation; hence the ductus venosus atrophies and becomes a mere ligament. The placenta, being now a foreign body in the uterus, is ex- pelled, and this organ, by the contractions of its walls, closes the ruptured and gaping vessels, thus providing against haemor- rhage. Coitus between the Sexes. In all the higher vertebrates congress of the sexes is essential to bring the male sexual product into contact with the Fig. 139.—Section of erectile tissue (Cadiat). o, trabeculee of connective tissue, with elastic fibers, and bundles of plain muscular tissue (c); &, venous spaces (Schaf er).. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Mills, Wesley, 1847-1915. New York, D. Appleton
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