40 OBSTETRICS. 



whilst the inner one (fig. 12,/,/,) remains as a distinct sac, to which 

 the name amnioyi is given. 



"The membrane thus formed embraces the embryo very closely at 

 an early period, and is continuous with the common integument of 

 the fcEtus, at the open abdominal parietes. At a later period it is dis- 

 tended with fluid, and so separated from the foetus, and after being 

 reflected upon the funis, of which it forms the outer coat, it termi- 

 nates at the umbilicus. It is thin and transparent, but of a firm 

 texture, resisting laceration much more than the other membranes. 

 Its external surface is somewhat flocculent, but internally it is quite 

 smooth, like serous membrane, and like it secretes a bland fluid." 

 This fluid resembles dilute serum, and is called liquor amnii. It 

 varies in amount from half a pint to several quarts, the average quan- 

 tity being about half a pound. 



It subserves several useful ends. It probably serves as nutriment 

 to the fcBtus during the early months ; it preserves an equable tem- 

 perature for it while remaining in utero, it protects it from the efi'ects 

 of sudden blows, shocks, &c. It is also useful in dilating the os 

 uteri, by protruding the membranes in the commencement of labour. 



The Chorion. — The outer membrane of the ovum is called the 

 chorion, and is formed during its passage through the Fallopian tube, 

 by receiving an additional layer of albuminous matter secreted from 

 the walls of the tubes, and this is surrounded by a fibrous membrane. 

 This new formation is one of great importance, as it is through this 

 the whole subsequent nutrition of the embryo is derived ; this is ac- 

 complished at first by means of a number of villous processes, which 

 proceed from the whole surface of the chorion and give it a rough, 

 shaggy appearance ; these villous processes serve as absorbing radi- 

 cles, drawing in the fluids supplied by the mother, until a more per- 

 fect communication is afforded by the placenta. As the ovum ad- 

 vances in age, these villi diminish in number, assume a vesicular 

 appearance, and finally disappear altogether, except at that part of 

 the chorion which is in contact with the uterus, and where the pla- 

 centa is subsequently formed. In some animals, this connexion be- 

 tween the villous coat of the chorion and the uterine surface is the 

 only one that exists, hence they are called n on -placental. 



The Placenta. — The formation of this organ commences by the 

 penetration of the villi of the chorion into the tubuli of the decidua 

 already described ; later we find a vascular connexion established 

 between them and the villi, by the agency of the allantois, in the 

 manner above mentioned, the allantois conveying the blood-vessels 

 of the foetus to that portion of the chorion. It must not, however, 

 be understood, that there is any direct communication between the 

 vessels of the foetus and those of the mother, the foetal tufls being 

 merely bathed in the maternal blood and drawing nourishment from 

 it by its own cells, which have the power of selecting, and of elabo- 



