VOL. LXXXVIII.] PHILOSOPHICAL TRANSACTIONS. 33Q 



of 1 auricle and 1 ventricle, as it is found to do in fishes. The junction between 

 the 2 ventricles is attended with a great advantage, in performing the circulation 

 through the placenta; where the length and convolution of the umbilical vessels, in 

 some animals, offer a great resistance to the force of the heart, and render more 

 exertion necessary. In the superior aorta, the circulation is carried on by the left 

 ventricle alone; as the ductus arteriosus does not join the aorta, till after the latter 

 has given off the carotid and subclavian branches. Vital air is communicated to the 

 blood of the embryo, as it is to the blood of fishes. This, in its passage through 

 the gills, is exposed to water, which is allowed by all to contain a large proportion 

 of vital or oxygen gas, and returns thence fitted to answer the purposes of life. 



In like manner, the blood of the mother, in the cells of the placenta, having 

 received the essential part of this gas from her lungs, is applied to the capillary 

 vessels of the umbilical arteries, which recei^ and transmit it to the embryo ; the 

 life of which so entirely depends on this communication, that an obstruction to 

 the circulation through the placenta, for 2 or 3 minutes, will sometimes irrecover- 

 ably destroy it. The gills of fishes form a permanent part of their bodies, because 

 they are designed to pass the whole of their lives in the same medium. This is not 

 the case in the embryo of viviparous animals; which, after birth, is to change its 

 situation for another, in which there is a direct exposure of the blood to atmospheric 

 air. For this reason, the placenta, whose use is only temporary, is attached to 

 the foetus by a slender connection, which is soon dissolved after birth. I have 

 thought it necessary to introduce the foregoing observations on the structure and 

 functions of the placenta, in order to show that the principal use of it is to transmit, 

 and apply respectively to each other, the blood of the foetus, and that of its mo- 

 ther. No other action is carried on by the vessels of the foetal portion of the pla- 

 centa, as far as is yet known, than what has been described, unless so much as may 

 be necessary for their own growth and nourishment. 



The tumour which gave occasion to this paper is however an instance to prove, 

 that these vessels are capable, like those in other parts, of forming solid organized 

 matter; and that very considerable deviations from the ordinary structure of the 

 placenta may exist, and be perfectly compatible with the life and health of the 

 foetus. Before the birth of a healthy child, an amazing quantity of liquor amnii 

 was evacuated, which was by accident received in a vessel, and was found to amount 

 to 2 gallons, Winchester measure. When the placenta came away, a hard solid 

 body was found in its substance. It was preserved by Mr. Mainwaring, under 

 whose care the case occurred, and was by him obligingly presented to me. Fine 

 injection was thrown into the arteries and vein of the funis umbilicalis; when they 

 were filled, they appeared to be enlarged thrice beyond their natural size. 



The placenta, thus prepared, was subjected to examination. Its anterior surface 

 was found to be covered with the amnion, behind which lay the chorion as usual. 

 Some branches, both of the arteries and veins, coming from the funis, ramified in 

 the common manner, forming the foetal portion of the placenta. Others, of a very 



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