The human uterus, in the unimpregnated state, consists of a succulent substance, in which we may perceive fibres running in every direction. In the interstices of these we find a serous fluid, which can easily be squeezed out. By injecting the vessels finely with size or mercury, we observe them to be numerously intermixed with the fibres, but very small. A portion of these vessels follow an irregular course toward the inner surface or cavity of the uterus, and open there upon the membrane which lines it. At the menstrual period the vessels enlarge or dilate a little, and their orifices become more distinct over the surface of the cavity, as may be ascertained by inspecting those who have died at this time. Sometimes a slight temporary serous secretion precedes the flow of the menses, and succeeds it. More frequently the vessels yield a bloody fluid at once, which continues a certain number of days.—When impregnation takes place, the vessels enlarge still more; and we can observe small trunks forming in the substance of the uterus, the largest of which are at the two sides where the spermatic and hypogastric branches meet[1]. The old fibres are more separated, and new ones added, especially at the fundus.—The quantity of interstitial fluid is also increased, so that the uterus becomes manifestly softer. The vessels, as they enlarge, tend to the uterine cavity; but instead of opening there, and yielding a fluid, as in menstruation, they either elongate themselves, or, which is more probable, form new and very delicate vessels, which project from the inner surface of the uterus, giving it an appearance as if covered with down. This takes place first and chiefly at the fundus, and, in a slighter degree, immediately above the narrow cervix, whilst the intermediate body remains still quite smooth. These vessels project for about a line in length, at right angles, from the surface which yields them; and as they consist of arteries and veins, the down, or efflorescence, which they form, has, after death, a striated or radiated appearance, the empty arteries being white, the fuller veins black or red. These vessels constitute the outer layer of the decidua, or what may be called decidua striata, although the stria be obliterated as gestation advances. Almost immediately after the formation of these primary vessels, they secrete from their extremities a membrane, or rather an irregular tissue of vessels, which, on account of their origin, may be called secondary. These assume a direction at right angles to those which formed them, so that they cross the stria or primary vessels, and therefore any body coming down through the first set of vessels, must, before it can get into the cavity of the uterus, either rupture this secondary organ, or push it before it. It is in this manner that the inner layer of decidua is formed, part of which is afterwards protruded before the vesicular ovum, constituting decidua reflexa, or protrusa.