390 A Manual of Veterinary Physiology. 



part passes from the right auricle into the left auricle 

 through a foramen in the auricular septum, known as the 

 foramen ovale ; from the left auricle it passes into the left 

 ventricle, and from here to the aorta, and thence over the 

 body, especially to the head and neck ; that portion of 

 blood which reached the right auricle by the anterior cava, 

 passes into the right ventricle in conjunction with the 

 balance of the blood received by the posterior cava, thence 

 into the pulmonary artery, a small portion passing on to 

 the lungs and behaving as in the adult, whilst the major 

 portion passes from the pulmonary artery direct into the 

 aorta by means of the ductus arteriosus, and is distributed 

 by means of the aorta to the posterior part of the body. 

 From the external iliac arteries arise the umbilical arteries, 

 which run along the sides of the bladder, pass out at the 

 umbilicus, traversing the substance of the cord to the 

 placenta, where the impure blood it conveys becomes re- 

 vivified by the maternal placenta, and in a purified con- 

 dition it returns once more to the body of the foetus by the 

 umbilical vein. 



Thus it will be seen that the blood circulating in the 

 foetus is a mixture of arterial and venous, arterial up to the 

 liver, where it mixes with the portal ; another mixture occurs 

 in the right auricle, but the greater part of that blood, as we 

 have seen, after performing work in the anterior extremities, 

 is sent by another channel to assist in the nutrition of the 

 posterior extremities, whilst the purest blood in the body is 

 principally directed to the head, which needs it the most. 



After birth the circulation changes : the ductus venosus 

 closes, the septum in the auricles becomes filled in, the 

 ductus arteriosus is obliterated, the umbilical arteries be- 

 come the lateral ligaments of the bladder, whilst the 

 umbilical vein becomes the round ligament of the liver. 



The Duration of Pregnancy for the mare is about 11 

 months, though it may vary within wide limits ; for the 

 cow, about J) months ; sheep and goat, 5 months ; pig, 

 4 months ; and bitch, 2 months. 



Among the changes which the uterus undergoes after 



