256 BREEDING 



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 whicli 

 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 con- 

 verging 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 Vjlood in the mature animal, while the 

 umbilical vein receives the renovated blood, and thus j^erforms 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 re- 

 ceives 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 ovcde) 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 ctrteriosus), which in the fcetus 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 



