54 



HUMAN ANATOMY. 



Fig. 63. 



Umbilical vein 



Remains of 

 V allantoic 



^-duct 



three constituents of the cord lie embedded within the delicate stroma formed by 

 the gelatinous connective tissue, l\i^ jelly of Wharton, surrounded externally by the 

 common amniotic investment. 



The details of the cord must necessarily vary with the period of gestation, 

 since the component structures undergo marked changes. On section of the funic- 

 ulus at the end of pregnancy, the following features may usually be distinguished : 



(i) The am7iiotic sheath, which is closely united with the underlying connective 

 tissue, except for a short distance beyond the umbilical opening, at which point the 

 amnion may be separated as a distinct layer. 



(2) The yV/Zy of Wharton forms the common ground-substance in which the 

 remaining constituents of the cord lie embedded. This tissue corresponds to the 

 mucoid type, and contains a generous distribution of stellate connective-tissue cells 

 which form a reticulum by their anastomosing processes. 



(3) The umbilical blood-vessels — two arteries and one vein — are the most con- 

 spicuous components of the cord, since their size increases with the demands made by 

 the growing foetus. The markedly tortuous umbilical arteries usually entwine the single 

 umbilical vein and slightly increase in lumen in their progress towards the placenta, 

 in the immediate vicinity of which an anastomosis very constantly is to be found. 

 Seldom in man, but always in certain mammals, as the mouse, the umbilical artery is 

 single. According to His, even the youngest human cords possess only a single 



umbilical vein, except in the immediate vi- 

 cinity of the placenta ; again, on entering 

 the body of the foetus the single vessel is 

 represented by two umbilical veins which, 

 for a time, course within the abdominal 

 wall. The right vein, however, soon un- 

 dergoes atrophy, while the left takes part 

 in the formation of the hepatic circulation. 

 Valves have been described within the um- 

 bilical vein. The latter shares with the 

 pulmonary vein the distinction of conveying 

 blood which has been oxygenated by respi- 

 ratory function. 



(4) The allantoic duct, as a distinct 

 canal, is usually obliterated by the third 

 month of foetal life ; at birth, however, 

 atrophic remains, consisting of a narrow 

 column of epithelial cells situated between the umbilical blood-vessels, are seen in 

 sections of the cord taken from the vicinity of the navel. 



The stalk of the vitelline sac, or umbilical vesicle, enclosing the vitelline duct 

 and supporting the vitelline, or omphalomesenteric, blood-vessels, is still present 

 during the second month ; at this period it lies within the extension of the coelom, 

 which is continued into the young cord. With the early disappearance of this space 

 the vitelline stalk and the associated structures disappear, and by the end of gesta- 

 tion usually all traces of these structures have vanished from the cord. The most 

 conspicuous details of the umbilical cord at birth, therefore, are the three umbilical 

 vessels, embedded within the gelatinous connective tissue and invested by the sheath 

 of amnion. 



The human umbilical cord is conspicuous on account of its exceptional length, 

 which averages from fifty to sixty centimetres, while measuring only about twelve 

 millimetres in thickness. The extremes of length include a wide range, varying from 

 twelve to 160 centimetres (four and three-quarters to sixty-three inches). 



The cord almost constantly exhibits a torsion, the spirals passing from left to 

 right when traced towards the placenta. In addition to the general twisting of the 

 cord, which begins towards the close of the second month, the umbilical arteries 

 display even more marked spiral windings, usually enclosing the somewhat less 

 twisted umbilical vein. The cause of this conspicuous torsion is probably to be 

 sought in the spiral growth of the umbilical blood-vessels, the twisting of the cord, 

 as well as the revolutions of the foetus, being secondary. 



Umbilical 

 artery 



-^' Remains of vitelline 



\ duct and vessels 



Umbilical artery 



Transverse section of umbilical cord of third month. 

 X 12. 



