On examining these vessels in an injected uterus to which the placenta is attached, we shall therefore find that all traces of a regular canal or tube are suddenly lost upon their entering the placenta; each vessel (whether artery or vein) abruptly terminating in a spongy cellular tissue. If a blow-pipe be introduced into a piece of sponge, we shall have a very simple but correct illustration of the manner in which the uterine blood circulates through the placenta. The cell into which each vessel immediately opens is usually much larger than the rest, so that when the cellular structure of the placenta is filled with wax, a number of irregular nodules[22] are found continuous with these vessels and passing into an infinity of minute granules, which are merely so many casts of smaller cells. That this cellular tissue pervades the whole mass of the placenta, and communicates freely with the uterine vessels by which it is filled with blood, is proved by repeating a very simple experiment of Dr. Hunter, viz. "if a blow-pipe be thrust into the substance of the placenta any where, the air which is blown into the cellular part opens, and rushes out readily by, the open mouths both of the arteries and veins." (Hunter, _op. cit._ p. 46.) That it also envelopes the umbilical vessels of the cord is shown by the fact, that if a pipe be inserted beneath the outer covering of the cord near to its insertion into the placenta, we shall be able to "fill the whole placenta uniformly in its cellular part, and likewise all the venous system of the uterus and decidua, as readily and fully as if we had fixed the pipe in the spermatic or hypogastric vein; so ready a passage is there reciprocally between the cells of the placenta and the uterine vessels." (_Ibid._ p. 47.) The maternal portion of the placenta therefore consists of a spongy cellular tissue, which is filled by the uterine vessels, and also of those trunks which pass through the decidua, and which form the communication between these vessels and the placental cells. [Illustration: _Foetal surface of the placenta._]