SEPARATION OF THE PLACENTA. 633 



In the human subject, as well as in most quadrupeds, the mem- 

 branes of the egg are usually ruptured during the process of par- 

 turition ; and the foetus escapes first, the placenta and the rest of 

 the appendages following a few moments afterward. Occasionally, 

 however, even in the human subject, the egg is discharged entire, 

 and the foetus liberated afterward by the laceration of the mem- 

 branes. In each case, however, the mode of separation and expul- 

 sion is in all particulars the same. 



The process of parturition, therefore, consists essentially in a 

 separation of the decidual membrane, which, on being discharged, 

 brings away the ovum with it. The greater part of the decidua 

 vera, having fallen into a state of atrophy during the latter months 

 of pregnancy, is by this time nearly destitute of vessels, and sepa- 

 rates, accordingly, without any perceptible hemorrhage. That por- 

 tion, however, which enters into the formation of the placenta, is, 

 on the contrary, excessively vascular ; and when the placenta is 

 separated, and its maternal vessels torn off at their necks, as before 

 mentioned, a gush of blood takes place, which accompanies or 

 immediately follows the birth of the foetus. This hemorrhage, 

 which occurs as a natural phenomenon at the time of parturition, 

 does not come from the uterine vessels proper. It consists of the 

 blood which was contained in the placental sinuses, and which is 

 expelled from them owing to the compression of the placenta by 

 the walls of the uterus. Since the whole amount of blood thus 

 lost was previously employed in the placental circulation, and since 

 the placenta itself is thrown off at the same time, no unpleasant 

 effect is produced upon the mother by such a hemorrhage, because 

 the natural proportion of blood in the rest of the maternal system 

 remains the same. Uterine hemorrhage at the time of parturition, 

 therefore, becomes injurious only when it continues after complete 

 separation of the placenta; in which case it is supplied by the 

 mouths of the uterine vessels themselves, left open by failure of the 

 uterine, contractions. These vessels are usually instantly closed, 

 after separation of the placenta, by the contraction of the muscular 

 fibres of the uterus. They pass, as we have already mentioned, in 

 an exceedingly oblique direction, from the uterine surface to the 

 placenta ; and the muscular fibres, which cross them transversely 

 above and below, necessarily constrict them, and effectually close 

 their orifices, immediately on being thrown into a state of contraction. 



Another very remarkable phenomenon, connected with preg- 

 nancy and parturition is the appearance in the uterus of a new 



