75. The umbilical arteries, two in number, are a continuation of the primitive iliac arteries; they pass over the lateral regions of the bladder, ascend along the internal surface of the anterior abdominal covering of the fœtus, approaching each other as they get nearer to the navel, through which they emerge in order to enter the sheath of the cord, winding round the vein in their way to the placenta. (77) The vein on the contrary which proceeds from the placenta is of a larger calibre than that of the two arteries taken together. It is soft and extensible—and after having slightly meandered within the umbilical sheath penetrates through the navel into the abdomen of the fœtus, directs its course towards the inferior surface of the liver, enters the antero-posterior fissure of that viscus, unites with the left branch of the hepatic and vena portarum, and reaches under the name of the venous duct, the inferior cava. It has no valves, except one at its intro-abdominal bifurcation. This vessel, throughout its course, becomes obliterated after birth. 76. The omphalo-mesenteric vessels consist of a vein and an artery, the ramifications of which, externally to the fœtus, are seen distinctly on the umbilical vesicle (vesicula intestinalis). They accompany the cord as far as the navel through which they pass into the abdomen. They then separate, the former proceeding on the right to join the trunk or one of the branches of the mesenteric vein, and the latter on the left to join the superior mesenteric artery. These vessels are obliterated, as the vesicula umbilicalis to which they belong disappear, and lastly disappear in their turn. They have, however, been observed sometimes in the cord of the full grown fœtus, like whitish solid filaments. (Chaussier, Beclard.) In the recent case of an individual twenty years of age, who died of consumption, these vessels were found as pervious as when existing in the embryonic state, and contained blood. (Spakenberg.)[16] 77. When the Ovum is advanced to the second or third month of its fecundated existence (for the period varies in different examples I have had under my observation); its attachment to the Uterus, is firm and complete. Many of the vessels of the external surface of the chorion, which by this time have increased in size, while their free ends have luxuriantly branched out in innumerable minute vessels, have clustered together and formed what are in reality cotyledons—by means of which the said attachment is principally effected. (Personal observations and experiments.) The congeries of these clusters of vessels constitute the principal part of what has been called the Placenta, which grows in size and expands as the gravid uterus expands progressively during gestation.