TIII-: K(i:r.\:. MEM Of MAN. 



al>out as thick as the little lingi-r ( 1 1 - 1 " nun. or n-f> inch). :u,.l &1 

 the considerable length of 50 to 60 cm. cjui'l inches). Italm<>-t 

 always exhibits a very pronounced t/tirtil tn-i.xt. which, regarded from 

 tin- embryo, runs usually from left to right. 



There are often knot-like thickenings of the umbilical cord, which 

 may be due to either of two causes. l-'or the most part they are 

 due to an i IK reaped growth here and there in the conn* < 

 matrix of t he cord (false knots). More rart-ly t hey are formed by 

 a knotting of the cord, which n->ults from the fact that the embryo, 

 in the motions which it executes in the amniotic fluid, accidentally 

 slips through a loop of the cord and then gradually tightens it into 

 a knot. The thickening then presents, in distinction from the other, 

 a true knot. 



The attachment of the umbilical cord to the placenta ordinarily 

 takes place in or near its middle (insertio centralis). However, 

 exceptions to the rule are not rare. Thus one distinguishes in addi- 

 tion an insertio marginalis and an insertio velamentosa. In the first 

 case the umbilical cord unites with the margin of the placenta; in 

 the second place it does not reach the placenta at all, but attaches 

 itself at a lesser or greater distance from the margin of the latter, 

 to the fostal membranes themselves, and sends out from that point 

 the outspreading large branches of its vessels to the placenta. 



Man is distinguished from almost all of the remaining Mammals 

 by the possession of a long slender umbilical cord. Its condition 

 in Man results from the great distension of the amniotic sac. 

 \Vhe: eas this at first lies close upon the body of the embryo, it sub- 

 sequently becomes so distended (compare fig. 144 with fig. 143) that 

 it fills the whole cavity of the blastodermic vesicle and everywhere 

 clings closely to the inner surface of the chorion. Owing to this, 

 the remaining structures the yolk >ac with its blood-vessels, the 

 .^lender canal of the allantois with its connective- tissue envelope, : id 

 the umbilical blood-vessels which emerge through the dermal navel 

 of the embryo into the extra-embryonic body-cavity and betake 

 themselves to the chorion, become more and more hemmed in by 

 the amnion, and finally are crowded together into a small cord. 



At first the umbilical cord is short, since it pursues a straight course 

 in unit ing the navel of the embryo to the foetal membranes; after- 

 wards it becomes greatly elongated and folded in the amniotic fluid. 



Its structure varies at different time> duni.ir pregnancy corre- 

 sponding to the changes which the yolk <ac and the allaiitoLs with 



their blood -vessels undergo. 



