In its primitive condition the heart is a mass of cells to which, as already described in the embryo, the vessels of the vascular area tend and ultimately reach, forming the rudiments of the circulatory system.

Cavities are constructed in the mass of cells representing the heart, which become separated to form the ventricles and auricles. Bloodvessels which were formerly only red lines acquire size and shape, and divide themselves into arteries and veins, and gradually the complicated mechanism which is described in the section on the anatomy of the organs of circulation is elaborated from a few clusters of cells.

Long before birth the foetus possesses a perfectly complete set of organs connected with the circulation, differing in a few details of construction to meet the peculiarities of the foetal environment. To understand the circulation of the blood in the unborn foal, it is desirable to refer to the description of the circulation of the blood in the adult horse (see p. 436, Vol. I, Anatomy section).

In regard to the foetal circulation, it will be convenient to commence with the umbilical arteries, two in number, which convey the blood which has already passed over the body of the foetus to the vascular tufts which constitute the placenta. In the way recently explained, the blood so conveyed effects an exchange of its effete matters through the walls of the foetal and the maternal capillaries, and receives in return nutriment and oxygen. Thus renovated, the blood is carried back by the converging capillaries, which unite to form the umbilical vein, which vessel with the two umbilical arteries and the urachus mainly constitute the umbilical cord. The blood in the umbilical arteries is really in the foetus comparable to the venous blood in the mature animal, while the umbilical vein receives the renovated blood, and thus performs the function of an artery.

Passing through the navel (umbilicus), the vein enters the liver of the foetus, and in the horse pours the whole of its blood into the portal vein. In animals other than soliped or single-hoofed, the vein divides before entering the liver, and sends part of its blood directly through a separate branch (the ductus venosus) into the posterior vena cava. In the equine foetus, however, all the blood gets into the vena cava at last, and thence to the right auricle of the heart, which cavity also receives the blood from the anterior part of the body through the anterior vena cava. This blood goes directly through the auricle into the right ventricle, while the blood from the posterior vena cava is directed by the Eustachian valve through an opening (the foramen ovale) in the muscular wall which divides the right from the left auricle, and at once passes to the left ventricle, and by the contraction of the walls of that cavity is driven over the body after having met with the blood in the right ventricle, which has passed into the pulmonary artery in the ordinary course, but instead of reaching the lungs has been diverted into the arterial duct (ductus arteriosus), which in the foetus leads directly from the pulmonary artery to the posterior aorta. It is of course understood that the foetal lungs are not respiratory organs, as no air can reach them; therefore nothing would be gained by the blood entering them in large quantity; in fact, that fluid has been aerated in passing through the placenta. After circulating over the body, the blood is again carried by the pulmonary arteries to the placenta, and the course of the circulation just described is repeated. The total result of the modification in the arrangement of the circulatory apparatus in the foetus is the distribution of mixed blood over the body; only that portion which passes through the umbilical arteries reaches the placenta and becomes oxidized and otherwise improved by the interchanges which take place between the maternal and foetal fluids.