310 



HISTOLOGY. 



The umbilical arteries arise within the embryo as the principal ter- 

 minal branches of the aorta; parts of them in the adult are called the 

 common iliac and hypogastric [internal iliac] arteries. They end in the 

 capillaries of the chorionic villi. The single umbilical vein is due to a 

 fusion of two; within the body only the left remains, passing from the 

 umbilicus along the under surface of the liver (as the ductus venosus) 

 to the vena cava inferior. 



The allantois which the umbilical vessels accompany, extends the 

 entire length of the cord as a slender tube or strand of cells. At birth it is 

 rudimentary but may be found usually between and equidistant from the 

 arteries. It is more conspicuous when Mallory's stain is used. Within 



FIG. 356. YOLK SAC AND PERSISTENT 

 VITELLINE VESSELS. EXPOSED BY 

 REFLECTING THE AMNION AT THE 

 DISTAL END OF THE CORD. 

 (Lonnberg.) 



FIG. 357. PART OF A HUMAN AM- 



NIOTIC VILLUS. X 330. 

 Ep., Epitrichium; S. C., stratum cor- 



neum; S. g., stratum granulosum; 



S. G., stratum germinativum; 



M. B., homogeneous layer; F. T., 



fibrous tissue: A. T., areolar tissue. 



the body the allantois dilates to make the bladder, and if its prolongation 

 into the cord remains tubular, urine may escape at the umbilicus (through 

 a "urinary fistula"). 



The yolk stalk, surrounded by an extension of the body cavity, is 

 found in young umbilical cords (Fig. 355, A). The loop of intestine 

 from which the yolk stalk springs may also extend into the cavity of the 

 cord, and if it has not been drawn into the abdomen at birth, umbilical 

 hernia results. If the cavity of the yolk stalk remains pervious the intes- 

 tinal contents may escape at the umbilicus (fecal fistula). Ordinarily the 

 stalk and its vitelline vessels, together with the coelom of the cord, have been 

 obliterated before birth and no trace of them remains in sections of the 

 cord. 



