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A MANUAL OF VETERINARY PHYSIOLOGY 



they are not yet prepared to receive the blood as they will be 

 after birth, when they become distended with air, and have taken 

 on their duties as breathing organs. The blood must therefore 

 take another course than through the lungs. This course is 

 provided by the ductus arteriosus (Fig. 253, d.a.), a short vessel 

 uniting the pulmonary artery to the aorta, and thus bringing 

 their lumina into communication. By this conduit the blood 

 enters the posterior aorta, and is conveyed to the hinder parts 

 of the body and to the placenta. 



The allantoic or umbilical arteries convey the blood from the 

 foetus to the placenta. These arteries are branches of the 

 internal pudics, or of the parent vessels, the internal iliacs, and 

 during intra-uterine life they are larger than the parent vessels. 

 Soon after birth, however, their walls become thickened, their 

 lumina are lost, and they become impervious to the passage of 

 blood. In the adult they are recognised as the thickened cords 

 found in the lateral ligament of the bladder. The ductus arterio- 

 sus just prior to birth has a lumen easily receiving an ordinary 

 cedar pencil, but it steadily diminishes until, at about a month 

 after birth, it is no greater than the diameter of a knitting-needle. 

 It is probafre that little blood passes this way after birth, but 

 the exact period of total occlusion is unknown. Similarly, the 

 foramen ovale is blocked up by the development of a membrane, 

 which may be pulled out with the forceps shortly after birth, 

 and then resembles in shape an old-fashioned lace nightcap or 

 cowl. When undisturbed, it lies in a heap, filling up the foramen. 



The short cuts in the fcetal circulation — viz., the ductus 

 venosus, ductus arteriosus, and foramen ovale — exist mainly with 

 the object of ensuring that the purest blood reaches those organs 

 which require it the most. The heart, head, and fore-limbs 

 receive blood which is much purer than the blood circulating 

 through the hind-limbs and abdominal viscera, for the brain 

 must be well fed. The fact is that the fcetal blood at its best 

 is far below the level of the arterial blood of the mother, and 

 this is explained by saying that the demand of the foetus for 

 oxygen is small, owing to the low rate of its metabolism. From 

 the blood of the umbilical artery and vein of the foetal sheep 

 the following gases have been extracted, and may be compared 

 with the arterial blood of the mother : 



