58 DR. LEE ON THE STRUCTURE OF THE HUMAN PLACENTA, 
On cutting into the placenta, he discovered in many places of its substance 
yellow injection, and in others red, and in many others these two colours 
mixed. The substance of the placenta, now tilled with injection, had nothing 
of the vascular appearance nor that of extravasation, but had a regularity in 
its form which showed it to be naturally of a cellular structure, fitted to be a 
reservoir for blood. 
From these appearances Mr. Hunter infers, “that the arteries which are 
not immediately employed in conveying nourishment to the uterus go on 
towards the placenta, and proceeding obliquely between it and the uterus, pass 
through the decidua without ramifying. Just before entering the placenta, 
after making two or three close spiral turns upon themselves, they open at 
once into its spongy substance, without any diminution of size and without 
passing behind the surface as above described. 
“ The veins of the uterus appropriated to bring back the blood from the 
placenta, commence from this spongy substance by such wide beginnings, as 
are more than equal to the size of the veins themselves. These veins pass 
obliquely through the decidua to the uterus, enter its substance obliquely, and 
immediately communicate with the proper veins of the uterus. This structure 
of parts points at once to the nature of the blood’s motion in the placenta. 
The blood detached from the common circulation of the mother moves through 
the placenta of the foetus, and is then returned back into the course of the cir- 
culation of the mother to pass on to the heart*.” 
Dr. William Hunter’s description of the vascular connexion between the 
uterus and placenta coincides with that of his brother : “ for it seems incon- 
testable (he observes) that the human placenta, like that of the quadruped, is 
composed of two distinct parts, though blended together ; viz. an umbilical 
which may be considered as a part of the foetus, and an uterine which belongs 
to the mother ; that each of these parts has its peculiar system of arteries and 
veins, and its peculiar circulation, receiving blood by its arteries and returning 
it by its veins ; that the circulation through these two parts of the placenta 
differs in the following manner:— in the umbilical portion the arteries terminate 
* Observations on certain Parts of the Animal CEconomy, by John Hunter, 1786 : page 127. 
