THE PLACENTA. 761 



The placenta, accordingly, is a double organ, formed partly by the 

 chorion and partly by the decidua ; and consisting of maternal and foetal 

 bloodvessels, entangled and united with each other. 



The part which this organ takes in the development of the foetus is 

 of primary importance. From the date of its formation, at about the 

 beginning of the fourth month, it constitutes the only channel through 

 which nourishment is conveyed from the mother to the foetus. The 

 nutritious materials, which circulate in the blood of the maternal 

 sinuses, pass through the intervening membrane, and enter the blood of 

 the foetus. The healthy or injurious regimen, to which the mother is 

 subjected, will accordingly exert an influence upon the child. Even 

 medicinal substances, taken by the mother and absorbed into the circu- 

 lation, may transude through the placental vessels, and thus exert a 

 specific effect upon the foetal organization. 



The placenta is, furthermore, an organ of exhalation as well as of 

 absorption. The excrementitious matters, produced in the circulation 

 of the foetus, are undoubtedly in great measure disposed of by transu- 

 dation through the walls of the placental vessels, to be afterward dis- 

 charged by the excretory organs of the mother. The sj^stem of the 

 mother may therefore be affected in this manner by influences derived 

 from the foetus. It has been observed in the lower animals, that when 

 the female has two successive litters of young *by different males, the 

 young of the second litter will sometimes bear marks resembling those 

 of the first male. In these instances, the influence which produces the 

 external mark is transmitted by the first male to the foetus, from the 

 foetus to the mother, and from the mother to the foetus of the second 

 litter. 



It is also through the placental circulation that those disturbing effects 

 are produced upon the nutrition of the foetus, which result from sud- 

 den shocks or injuries inflicted upon the mother. There is little room 

 for doubt that various deformities and deficiencies of the foetus, confor- 

 mably to the popular belief, originate, in certain cases, from nervous 

 impressions experienced b}* the mother. The mode in which these effects 

 may be produced is readily understood from the anatomy and functions 

 of the placenta. It is well known how easily nervous impressions will 

 disturb the circulation in the brain, the face, or the lungs; and the 

 uterine circulation is quite as readily influenced by similar causes, as 

 shown in cases of amenorrhcea and menorrhagia. If a nervous shock 

 may excite premature contraction in the muscular fibres of the pregnant 

 uterus and produce abortion, it is undoubtedly capable of disturbing 

 the circulation of the blood in the same organ. But the foetal circulation 

 is dependent, to a great extent, on the maternal. The two sets of vessels 

 are united in the placenta, and as the foetal blood has much the same 

 relation to the maternal, that the blood in the pulmonary capillaries has 

 to the air in the pulmonary air-cells, it must be liable to derangement 

 from similar causes. And lastly, as the nutrition of the foetus is pro- 

 vided for wholly by the placenta, it will suffer from any disturbance of 

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