Circumference. — The mar.in of the pla centa is bordered all rounds by the united membranes vvhich enter into its composition. Here the amnion and chorion, after lining the fcetal or concave surface, come into contact with the decidua which covers its uterine face, and the three membranes then pass off to gether to enclose the liquor amnii and fcetus. At this part the decidua is always most dense.
Partly within its substance is formed an in complete sinus, the circular vein or sinus. This constitutes an interrupted channel, which more or less encircles the placenta. Several orifices are observed in its walls. Some com municate directly with the interior of the pla centa, and others with the uterine sinuses.
Substance. —When a clean section has been made through the placenta (fig. 484.), the two surfaces already described are observed to enclose between them a soft spongy sub stance, which is made up principally of count less ramifications of the fcetal villi. These are attached at their base to the chorion, from which they spring, while their opposite extre mities are united to the deridual layer form ing the uterine boundary of the placenta. The interspaces left between the villi and their ramifications forni what have been termed the cells of the placenta. They are widest between the roots of the villi, and much smaller between their extremities. In these spaces the maternal blood circulates. When injections are thrown into the placenta from the uterine arteries or veins, these spaces become filled, and the mass, when broken, exhibits a peculiar granular appearance. Dip ping down among the villi, and reaching in some cases as far as the fcetal surface of the placenta, are numerous sheet-like prolonga tions of decidua (fig. 484. dp). These con stitute the dissepiments which separate the entire mass into its several lobes or cotyle dons. At the placental margin, the decidual layer generally dips under the villi, forming a return end or border, which is directed in wards, and is attached at a distance of 3-4'" from the margin to the outer surface of the chorion. The exact relation of the decidua to the villi, in various parts of the placenta, will be better understood after a more minute description has been given of each of these structures.
The tufts and villi. — A placental tuft has been often compared to a tree. It con sists of a trunk giving off numerous branches, which ultimately end in finer subdivisions or villi (fig. 484. and dig. 485. a). The
trunks may be said to take root in the cho rion, from which they spring, while the branches and finer subdivisions spread la terally and upwards, until they- come into contact, at their sides, with the adjacent tufts and villi, and above with the decidua which bounds the placenta towards the uterus. Many of the villi, instead of branching like trees, proceed thread-like from the floor to the roof of the placenta, only sending off' short knotty side branches. The tufts are so closely set, that their forms cannot be readily discerned until they are floated out in water. The steins are tough and fibrous, or coriaceous, while the branches and finer villi, though strong, are of a more brittle texture. When one of these is broken off; and examined by the microscope, it presents the following characteristics — the subdivi sions are abrupt, contorted, and singularly devoid of symmetry ; from all parts of their surface spring numerous short pullulations. which render them knotty and uneven.
Every villus is composed of two distinct parts, viz. an outer leathery sheath, and an inner softer and vascular structure, which is contained within the former like a fin.er en cased in a glove. The distinction between these two structures is not easily observed, except in parts where the outer sheath has been accidentally broken off, leaving the more pulpy internal substance exposed. Or in cases where the placenta has become stale by keep ing for a few days, when the inner portion by shrinking has retired from the end of the vil lus, so that a small interspace has been here left (fig. 485. b).
When a terminal tuft so prepared is viewed by transmitted light, under slight compression, the outer case is seen to consist of a trans parent non-vascular structureless membrane, embedded in the substance, or attached to the inner surface of which are numerous flattened spheroidal cells, forming generally a single layer. In the apex of a growing tuft, or forming a distinct bud projecting from its extremity, may be often observed a group of similar cells which appear to be passing off from a spot in the centre of the mass.* These cells perform important parts in the growth and offices of the villi, which will be presently noticed.