Examination of the lumen of the umbilical artery in the constricted state shows it to be very narrow and in the form of the letter Y; this comes from the inward bulging of the vessel walls as a result of the contraction of its longitudinal fibres; this contraction almost com pletely closes the lumen of the vessel, an open lumen being found for only short distances. This, however, only holds good for the umbilical arteries of children born viable, whereas these bulgings never occur in the arteries of still-born infants. The umbilical vein forms a smooth wide tube and possesses likewise a single layer of endothelium; its mus culature runs in various directions but is for the most part circularly disposed. There is a band of elastic fibres beneath the endothelium (Bondi); the muscular coat is traversed by wavy connective tissue and by a few elastic fibres. 'In its extra-abdominal portion the umbilical vein also possesses a well developed adventitia of embryonic connective tissue, which is wanting in its abdominal portion, where the vessel lies rather free in its surroundings (Herzog).
At the moment of birth as a consequence of the opening of the pul monary circulation the blood pressure in the aorta and in the entire greater circulation, including the umbilical arteries, falls; for now, in stead of both sides of the heart, only the left side is active in pumping the blood through the systemic circulation. The umbilical arteries grad ually contract throughout their extent: the contraction commences in the peripheral portion and extends to the abdominal portion and is materially assisted by the mechanical irritation arising from the cutting of the cord and perhaps also by its cooling; thus the arteries are so well contracted that in the vast majority of cases no bleeding takes place after the cutting of the cord even should no ligature he applied. The vein also contracts: its contained blood flows out, and under normal conditions after birth it is either empty or contains only a thin clot in the neighborhood of the navel. Since the cord, as stated, possesses no blood vessels of its own for its nourishment its death begins with the cessation of the placental circulation. This occurs normally through a process of mummification, which is assisted by all factors promoting evaporation, especially by warmth and dry air.
The desiccation begins at various points on the cord and is as a rule completed by the third day. Then the cord is converted into a dark, ropy, flat body; the drying out process takes place throughout the cord except at its base and here the cord stays moist over a few milli metres of its extent and disintegrates gradually. Simultaneously changes occur in the navel, corresponding to a demarcating inflammatory process; these cause the separation of the cord. The cylinder of skin which covers the lower part of the cord swells and becomes red; its border frees itself slightly from the cord and begins to roll in upon itself; in this way there is formed between the cylinder of skin and the desic cated cord, a furrow which is filled with greasy material composed of tissue detritus, pus cells, and bacteria. On histological examination the capillaries of the network under the skin covering the cord are widened and collections of leucocytes are found in the skin and in the jelly of Wharton. The cord gradually loosens at its base, remaining longest attached at the blood vessels. Occasionally the stumps of the vessels protrude several millimetres out of the base of the wound after the cord has separated. Meanwhile the infolding of the skin-cylinder progresses gradually and after the separation of the cord, the ,small skin wound, usually covered with a secretion or crust, lies in a little funnel. At the
same time epidermization starts at the periphery and becomes com plete about three weeks after birth.
Shortly after birth the process of involution begins in the abdom inal portion of the umbilical vessels. The peripheral portions, at the navel itself, become involved in the inflammatory processes taking place at the navel; their walls are invaded by round cells and their lumina closed by small-celled infiltration. The thrombi, which are reg ularly found in the arteries seldom form in the veins. The intima prolif erates and gradually becomes converted into connective tissue (arteritis obliterans). In the third week of life degeneration of the media occurs; the muscularis seems shredded and is permeated with capillaries, its structure becomes indistinct and the nuclei stain feebly; later the mus cular coat disappears, being replaced by connective tissue. The veins and arteries usually remain passable for a thin sound for months and occasionally for years after birth.
The adventitia of the umbilical arteries furnishes the material for the formation of a dense connective tissue and this attaches the arteries, as well as the connective tissue strands resulting from their involution, to the navel ring and to its lower half. Thus the lower half of the ring is closed by a strong buttress of connective tissue, the fibres of which ex tend into the skin of the navel. Conditions are however different in the upper half of the navel ring. Only a few- loose strands of connective tissue form around the umbilical vein. The vein, after its fibrous meta morphosis, draws over to the connective tissue bolster which is formed from the adventitia of the arteries and thus there ensues a defect in the upper half of the navel, between the upper half of the ring and the con nective tissue strands of the vein which are drawn over to the arteries; this defect is covered underneath, toward the abdomen, by only the thinnest of connective tissue layers and thus offers a place of lessened resistance the impact of the abdominal contents on crying or straining. Herzog designates this defect the "canalis umbilicalis." Some writers teach that the media and intitna of the umbilical vessels retract, so that in the upper portion, lying nearest the navel, only the adventitia changed into connective tissue, remains attached. This retraction, however, is disputed by well-known authorities (Herzog, Kot kel).
After its involution the navel appears as a defect in the abdominal wall, closed by connective tissue tightly in its lower half, insufficiently in its upper portion. The fascia tonbilicalis (the portion of the transver sails fascia lying underneath the navel) extends beneath the connective tissue and below this is peritoneum. The umbilical vessels are changed to tense fibrous bands. From the vein, the ligament um tcres is formed, passing along the anterior abdominal wall in the free margin of the fal cifortn ligament of the liver until it reaches the median incisu•e. That part, alone, of the umbilical vein which connects with the left branch of the portal vein normally remains patent, and persists as a branch of the portal vein, being traversed by the blood in a opposite direction to that in ftetal life.
The umbilical arteries, after their obliteration and conversion into connective tissue, form the lateral umbilical ligaments, which extend from the latent] pelvic wall to the anterior abdominal wall and running properitoneally pass to the umbilicus.