Embryo of the Sheep.

Fig. 423. Embryo of the Sheep.

(2489). The appearance of the placenta varies much in different tribes: thus in the Sheep and other Ruminants it consists of numerous detached masses of villi (fig. 423, i i), that interdigitate with corresponding processes derived from the maternal womb; in the Mare it covers the whole surface of the chorion; but in the greater number of Mammals, and in the Human female, it forms a single vascular cake, whence is derived the name appropriated by anatomists to this important viscus.

(2490). After the development of the placental system, it is obvious that the arteries derived from the common iliac trunks of the foetus, which at first were distributed only to the allantois, as in the case of the Bird (§ 2139), on the development of the placenta become transferred to the latter viscus, and form the umbilical arteries of the navel-string. The vein, likewise, notwithstanding its prodigiously-increased extent of origin after the placenta has been formed, takes the same course on entering the umbilicus of the foetus as it did when it was derived only from the allantois; so that, although the placenta completely usurps the place of the allantois, both the allantoic and placental circulations are carried on through the same umbilical arteries and veins.

(2491). In order to complete our history of foetal development up to the full establishment of the permanent double circulation that characterizes all the hot-blooded Vertebrata after birth, it only remains for us to notice the changes that occur in the vessels of the foetus, whereby, on the cessation of the functions of the placenta, the pulmonary circulation is at length brought into action.

(2492). Up to the period of birth the arrangement of the foetal circulation remains essentially that of a Reptile, inasmuch as both the venous blood derived from the system and the arterialized blood that comes from the placenta are mixed together in the as yet imperfectly separated chambers of the heart. Under these circumstances the arrangement of the vascular system is as follows: - Pure blood, supplied from the placenta, is brought into the body by the umbilical vein, which passes partly into the portal system of the liver, but principally through the ductus venosus into the inferior cava, and thence into the heart. From the construction of the heart during this portion of foetal existence it is obvious that in that viscus all the blood derived from the placenta, from the venous system of the foetus, and also from the as yet inactive lungs, is mingled together prior to its distribution through the arterial system. The two auricles communicate freely with each other through the foramen ovale; and, by means of the ductus arteriosus, the greater portion of the blood driven from the right ventricle during the systole of that cavity passes into the aorta, a very small proportion only finding its way into the pulmonary arteries.

Such a heart, therefore, supplies to the foetal system a mixed fluid, of which a portion, having passed through the arterial trunks, finds its way back to the placenta through the two umbilical arteries, there to recommence the same circle.

(2493). Immediately after birth, however, the whole arrangement is altered, and the adult condition fully established. The lungs assume their functions and the pulmonary arteries attain their full proportions, while the placenta at once ceases from its office, and all the umbilical vessels become obliterated. The ductus venosus is no longer permeable, so that the portal system and that of the venae cavae are quite separated: the foramen ovale closes, thus completely separating the right from the left auricle: the ductus arteriosus is reduced to a mere ligament; all the blood, therefore, driven from the right side of the heart must now pass into the expanded lungs, and be returned through the pulmonary-veins to the left side of the heart. Thus the pulmonary and systemic circulations being rendered totally distinct, arterialized blood alone enters the arterial system, to be distributed through the body, and, the umbilical arteries disappearing, the highest form of the circulatory apparatus is fully established.

(2494). After birth the mammary glands supply the first nutriment to the still helpless offspring. These vary in number and position in different species of placental Mammifers, their number being of course greatest in the most prolific races. Where the arms or anterior limbs can be used for supporting or clasping the feeble young, as in the Quadrumana, the Bats, and the females of our own species, it is upon the breast that these nutrient founts are placed; but in less gifted tribes the mammae are situated beneath the abdomen or in the inguinal region. Their structure, however, is similar throughout the entire class; each gland consisting of innumerable minute secreting cells, grouped together in lobules and in lobes. Delicate excretory ducts, derived from all these ultimate cells, unite together again and again until they form capacious ducts, or rather reservoirs for milk. In the Human female the lactiferous canals terminate by numerous orifices upon the extremity of the nipple; but where the nipples are of large size, they generally contain a wide cavity wherein the milk accumulates in considerable quantities, to be discharged through one or two orifices only.

Such are the modes by which Supreme Beneficence has provided for the infant progeny of Mammiferous beings, and conferred the endearments of maternity where He has bestowed intelligence to appreciate affection. But even this is not all: from the superabundance of the store provided there may be yet to spare; and Man is privileged to bid his lowing herds yield him their milk for food, and thus obtains no slight addition to the bounteous table spread for his enjoyment.