In examining the uterine surface of a full grown placenta it is necessary to place it upon something convex, in order that it may resemble, as nearly as possible, the form which it had when attached to the concave surface of the uterus; the cotyledons are thus rendered prominent and separated from each other; the sulci, which run between them, are wide and gaping: whereas, when the placenta is laid upon a flat surface, its cotyledons are closely pressed together, and the sulci more or less completely concealed. On minute examination of these sulci a number of openings may be observed, varying in size and shape, but usually more or less oval, their edges distinct, smooth, and thin; on directing a strong light into some of the larger ones a number of smaller apertures may be observed opening into them, in much the same way as is observed when looking down a large vein. Some of these canals do not immediately lead to smaller orifices as above described, but open at once into an irregular-shaped cell or cavity, in the parietes of which numerous small apertures may be observed, through which blood oozes when the adjacent parts of the placenta are slightly pressed upon. Besides these openings at the bottom of the interlobular sulci, others may be seen here and there upon the cotyledons; these are generally smaller, their edges thicker, and in most instances they are round; but they are not so invariably met with as the openings between the cotyledons, these lobular projections being sometimes very thickly covered with placental decidua. The openings observed on the uterine surface of the placenta correspond to the mouths of the uterine veins and arteries, which, in the unimpregnated state, open into the cavity of the uterus, but which now, by means of the decidua, convey maternal blood to and from the placenta. "Any anatomist," says Dr. W. Hunter, "who has once seen and understood them, can readily discover them upon the surface of any fresh placenta; the veins, indeed, he will find have an indistinct appearance from their tenderness and frequent anastomoses, so as to look a good deal like irregular interstitial void spaces: the arteries which generally make a snake-like convolution or two, on the surface of the placenta, and give off no anastomosing branches, are more distinct." (Hunter, _op. cit._ p. 46.) From the observations of Messrs. Mayo and Stanley, and from their examination of the original preparations in the Hunterian museum at the College of Surgeons, London, illustrating this subject, it appears that, in all probability, most of the large thin-edged apertures at the bottom of the interlobular sulci are connected with the uterine veins; whereas, the smaller orifices, the margins of which are thicker, and which are chiefly observed upon the cotyledons, are continuations of the uterine arteries.