CHAPTER XI. 

 THE PLACENTA. 



IX man and mammalians the embryo, during mtra-uterine life, is 

 dependent upon the uterus for the materials of its growth ; and this 

 supply of nourishment is provided by means of two vascular mem- 

 branes. One of these membranes, the chorion or allantois, is an out- 

 growth from the embryo ; the other is the mucous membrane of the 

 uterus. By their more or less intimate juxtaposition, the fluids tran- 

 suded from the blood-vessels of the mother are absorbed by those of the 

 embryo, and a transfer of nutriment thus takes place from the maternal 

 to the foetal organism. 



In some animals, the CQnnection between the maternal and foetal 

 membranes is a simple one. In the pig, the uterine mucous membrane 

 is everywhere uniformly vascular ; its only peculiarity consisting in 

 the presence of transverse folds, projecting from its surface, like the 

 valvulae conniventes of the small intestine. The surface of the allan- 

 tois is also smooth and uniformly vascular. No special development 

 occurs at any part of these membranes, and no adhesion takes place 

 between them. The vascular allantois of the foetus lies in simple appo- 

 sition with the vascular mucous membrane of the uterus, each of the 

 contiguous surfaces following the undulations of the other; and the 

 transudation and absorption between the two sets of blood-vessels pro- 

 vide for the nutrition of the foetus. When parturition takes place, a 

 moderate contraction of the uterus is sufficient to expel its contents ; 

 the egg being displaced from its position and discharged from the uterus 

 without hemorrhage or laceration of the parts. 



In other instances, there is a more intimate connection, at certain 

 points, between the foetal and maternal structures. In the cow, sheep, 

 and ruminating animals generally, the external membrane of the egg, 

 beside being everywhere supplied with blood-vessels, presents, scattered 

 over its surface, numerous rounded or oval spots, covered with thickly 

 set, tufted, vascular prominences. These spots are called cotyledons, 

 or cups, because each one is surrounded by a rim or fold, which embraces 

 a corresponding mass projecting from the inner surface of the uterus. 

 This portion of the uterine mucous membrane is also abundantly sup- 

 plied with blood-vessels; and the vascular tufts of the foetal membrane 

 are entangled with those belonging to the uterus. There is no absolute 

 adhesion between the two sets of vessels, but only an interlacement of 

 their ramified extremities ; and by careful manipulation the foetal por- 



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