330 Veterinary Obstetrics 



the left ventricle. From this latter cavity, it is driven along the 

 common aorta until it reaches the carotid and subclavian arteries, 

 through which it is carried almost wholly to the head and an- 

 terior limbs. 



While the aorta is freely open from the heart along the poster- 

 ior aorta toward the posterior portions of the body, it seems that 

 very little of the blood from the left ventricle passes backward. 

 This is largely because the blood from the right ventricle, 

 which, at this period, is as strong as the left, has already filled 

 that portion of the posterior aorta posterior to the juncture of 

 the ductus arteriosus with that vessel. Consequently, the blood 

 pressure in the two portions of the vessel is approximately equal, 

 so that there is as great a tendency for the blood from the right 

 ventricle to pass forward from the ductus arteriosus as for that 

 from the left to pass backward from the opening of the ductus 

 arteriosus when propelled through the common aorta. 



In some cases, it has been found that the aorta has become 

 obliterated during embryonic life at a point just anterior to its 

 juncture with the ductus arteriosus and posterior to the fourth 

 aortic arch, so that all the blood to the posterior end of the fetus 

 must pass through this vessel. This condition of independent 

 anterior and posterior circulations has not interfered with the 

 development of the fetus, but, at the time of birth, the circulation 

 is at once blocked to all the posterior portions of the body, so that 

 the new-born young must promptly perish. 



The plan of the fetal circulation is in a measure the reverse of 

 that after birth, the purified or red blood coming from the pla- 

 centa along the posterior systemic veins to the right auricle, 

 thence passing largely to the head, neck and anterior limbs 

 through the right ventricle, ductus arteriosus, carotid and sub- 

 clavian arteries. This would suggest that the head received 

 purer blood of higher nutritive value and that, consequently, the 

 head end of the fetus should develop most rapidly. In the earlier 

 stages of fetal life, this apparently holds true but later, in the 

 larger herbivora at least, the development of each of the two 

 ends of the body becomes approximately equal. Throughout 

 fetal life, the blood of the entire body is of a mixed character, 

 the red blood of the umbilic veins becoming mixed with venous 

 blood before reaching the heart. 



