CHANGE IN THE CIRCULATION AT BIRTH. 979 



driven into the corresponding ventricles, it follows that the partially 

 arterialized blood in the left auricle enters the left ventricle, and from 

 it is propelled into the aorta and its branches, whilst the venous blood 

 in the right auricle enters the right ventricle, and from it is propelled 

 into the pulmonary artery. The pulmonary artery is, in the embryo, con- 

 nected by a short arterial trunk, named the ductus arteriosus, with the 

 under side of the arch of the aorta. Hence, the blood in the descending 

 aorta, below that point, is mixed with the venous blood collected in the 

 right auricle, whilst the arch of the aorta contains the arterialized 

 blood collected in the left auricle. This latter blood is therefore con- 

 veyed by the great branches given off from the arch of the aorta to 

 the anterior part of the embryo, i. e., to the head, neck, and upper 

 limbs ; whilst the former, or non-arterialized blood, proceeds, in very 

 small quantity, through the still narrow pulmonary artery into the 

 collapsed lungs, but almost entirely through the descending aorta, 

 iliac arteries and their branches, to the posterior part of the trunk of 

 the embryo and the hinder limbs. A portion of it, however, is also 

 conveyed along the hypogastric or umbilical arteries to the allantois, 

 or placenta, to be once more renovated ; from these parts it is then 

 returned to the embryo by the umbilical vein, as arterialized blood, 

 and pursues the course already described over again. The foetal cir- 

 culation is, therefore, largely accomplished by the action of the right 

 ventricle, and the walls of this cavity, before birth, are of equal thick- 

 ness with those of the left ventricle. The right auricle is then also 

 larger and thicker than the left. 



It is probable that, from the admixture of the venous blood of the 

 embryo, at several points, with the arterialized blood brought from 

 the allantois or placenta, no portion of the embryo is supplied with 

 pure arterial blood, especially whilst the foramen o-vale, between the 

 auricles, is very patent. Nevertheless, the head and upper limbs al- 

 ways receive blood more perfectly oxygenated than that distributed to 

 the lower half of the body. This difference is associated with the more 

 rapid development of the important parts situated in the anterior half 

 of the embryo. Thus, the head, the encephalon, and the organs of the 

 senses, exhibit a far greater activity of development than the hinder 

 portion of the embryo and the corresponding part of the nervous axis. 

 The anterior limbs also show a greater relative development than the 

 posterior. These conditions have even been regarded as having the 

 relation of cause and effect ; but probably they are merely associated 

 conditions. As the heart approaches its perfect state, and the fora- 

 men ovale becomes smaller, and its course through the auricular sep- 

 tum more oblique, the arterialized blood is probably more completely 

 conducted into the left auricle, and so, through the left ventricle, 

 aorta, and great arteries, reaches more- especially the anterior part of 

 the body. 



Change in the Circulation at Birth. 



At the period when the young Reptile escapes from its egg, when 

 the young Bird first chips the shell, and when the young Mammal is 



