The Placenta 365 



with sufficient rapidity to accomodate the rapidly growing 

 visceral mass and, consequently, a portion of it is crowded out 

 through the umbilical opening alongside the vessels and remains 

 outside of the abdominal cavity until the contraction of the 

 umbilicus gradually forces the intestines back into the now more 

 capacious abdomen where they permanently remain. In numer- 

 ous cases the umbihc ring does not normally contract, the ring 

 remains open, the intestinal loop fails to return within the 

 abdominal cavity and the fetus is born with umbilic hernia. 



The vessels of the umbilic cord are disposed spirally from left 

 to right, a condition sometimes suspected to be due to a re- 

 volution of the fetus upon its short axis but apparently more 

 properly referable to a fundamental plan in development unin- 

 fluenced by fetal movements. The spiral arrangement of the 

 cord adds to its compactness and strength and to the maintenance 

 of the vessels in regular order. 



The length of the umbilic cord varies widely with species and 

 individuals. In the mare the length ranges from 35 to 40 inches, 

 the amniotic portion representing about one third of the total 

 length. It is sufficiently long that, when a foal is normally 

 born and the dam is in the recumbent position, the cord remains 

 intact and may even persist after the mare has risen to her feet, 

 but ruptures when she turns her head to the foal, to lick it, or 

 starts to move away from it. Sometimes the foal, by its strug- 

 gles, ruptures the cord by its own efforts. In other cases, the 

 cord remains intact and the chorion is completely expelled, still 

 attached to the living foal, and its rupture takes place after the 

 foal rises to its feet. 



The point of normal rupture of the navel cord of the foal is 

 about ij4 to 2 inches from the umbilicus, at a well marked con- 

 striction. This delimits the cord proper from what may be called 

 the umbilic base, or cutaneous navel, a hairless, cylindrical, firm 

 segment i to i}4 inches long. At the termination of this por- 

 tion a softer and more fragile cord begins with a somewhat con- 

 stricted neck and, within this latter at a point ^ inch or more from 

 the base, the rupture of the cord generally occurs. (See Fig. 

 90 B.) 



The navel cord of the cow consists of an amniotic portion only 

 and is less than }i the length observed in the mare, ranging be- 



