CH. LVIII.] 



THE FCETAL MEMBRANES. 



823 



them at first from the fluid matter, secreted by the glands of the 



uterus in which they are soaked. Soon, however, the foetal 



vessels of the villi come into more intimate relation with the 



vessels of the uterus. The part at which this relation between 



the vessels of the foetus and those of 



the parent ensues, is not, however, over 



the whole surface of the chorion ; for, 



although all the villi become vascular, 



yet they become indistinct or disappear 



except at one part where they are greatly 



developed, and by their branching give 



rise, with the vessels of the uterus, to 



the formation of the placenta. The 



structure and functions of the placenta, 



however, we have already described in 



connection with the decidua. 



The umbilical cord is composed 

 of the following parts: (i.) Ex- 

 ternally, a layer of the amnion, re- 

 flected over it from the umbilicus. 



(2.) The umbilical vesicle or yolk-sac with its duct and ap- 

 pertaining omphalo-mesenteric blood-vessels. (3.) The remains 

 of the allantois, and continuous with it the urachus. (4.) 

 The umbilical vessels, two arteries and one vein, which ultimately 

 form the greater part of the cord. These are embedded in a jelly- 

 like connective tissue called the Whartonian jelly. 



The After-birth. In parturition, the pressure of the uterine 

 and abdominal walls upon the uterine contents, and especially on 

 the amniotic fluids, causes a bulging of the membranes (combined 

 decidua}, chorion and amnion) through the os uteri. When the 

 membranes are ruptured the fluid escapes, and then the foetus is 

 expelled. Later contractions of the uterus detach the placenta 

 from the uterine wall, and this is in turn expelled ; the separation 

 extends around the decidua lining the rest of the uterus, and, 

 turned inside out, this follows the placenta, carrying with it the 

 other membranes. This constitutes the after-birth. ' The sever- 

 ance of the umbilical cord should not be done until some minutes 

 after the birth of the child, or it is deprived of a good deal of 

 the blood which is subsequently squeezed out of the placenta 

 into it. 



