The internal portion (fig. 485. b) consists of a soft and pulpy structure which envelopes the blood-vessels of the villi. In its substance also are embedded numerous cells of a similar nature to those observed in the structure less sheath.
Termination of the firtal vessels.— The ar rangement and terminal divisions of the blood vessels within the villi varies considerably according to the age of the placenta. The following distribution is observed from the third to the sixth month (fig. 485. a). Each villus contains one or more arteries and veins, together with numerous capillaries. The ar teries pass up the centre of the stem, and divide into branches according to the number of the terminal subdivisions. Within these the branches split up into numerous capilla ries, which present various forms of arrange ment, in some parts resembling Malpizhian bodies, and in others the arrangement of pul monic capillaries. From these capillaries the blood is collected by veins which pass back through the tufts accompanying the corre sponding arteries. All these vessels, with their subdivisions, are enveloped and sup ported by the pulpy granular substance that forms the interior of every villus (fig. 585. b).
Towards the end of preanancy, the true capillaries of the villi gradua'lly disappear, so that in a placenta at term the blood-vessels present the condition accurately described by C. II. Weber and Goodsir. A single vessel generally enters each terminal tuft, and after forming an open loop, it returns again, either dividing within the villus, or leaving it as it entered. Or a single vessel may enter, and retire from two or more villi, before it termi nates in a principal vein. Many modifications occur in the forms of the loops, which inay be simple, compound, wavy, or much contorted, and in parts varicose.* Such, then, are the structures belonging to the fcetus which are brought into contact with the maternal blood in the interior of' the pla centa, viz. the portion of chorion that forms the floor of the placenta, and the tufts or villi which spring from its surface. The office of the former is simply mechanical in confining the maternal blood to its proper course, and preventing rupture of the organ ; the latter constitutes the potential portion of the pla centa.
On the other hand, the sole parts belonging to the mother, the existence of which can be anatomically demonstrated in the substance of the placenta, are formed out of the decidua.
The decidua.—A general description of this membrane, as it forms the roof of the placenta, and sends off dissepiments into its substance, has been already given. It only remains to explain the exact relations of this structure to the villi, within the placenta. All the extre mities of the villi which are sufficiently long to reach across the placenta from the chorion to the opposite surface formed by the decidua, become firmly attached to the inner side of the latter. This attachment takes place not by any actual perforation of the decidua, but by the ends of the villi being simply inserted, in an early stage of the formation of the pla centa, into little shallow pits or cup-like de pressions in the decidual substance, into which they are received, and from which they may be withdrawn.* In other cases, the ends of the villi become blended with the decidua, to which they are apparently fixed, by a growth of decidual cells. These attachments are for the purpose of giving strength to the placenta, and of mechanically supporting the villi. They take place not only between the ends of the villi and the decidua forming the roof of the placenta, but also wherever decidua and villi come into contact. Hence similar attachments are also formed between the villi and the septa or dissepiments (fig. 484.. d p), NS hich divide its substanceinto separatelobes. Upon thefloor also of the placenta all round the margin, where the decidua turns downwards and inwards to beconie united with the chorion, and to form the placental margin, the decidua is found for a short distance attached to the bases of the villi. And this arrangement gives to the parts an appearance as if the decidua had been here penetrated by the villi, but one which is actu ally occasioned by the former having, in the course of growth, become extended around the roots of the latter long after these were first formed. Occasionally also decidual cells may be found upon the surface of villi, con necting together their extremities, or forming here and there rough irregular belts upon their steins.