118 ANATOMY OF THE RABBIT 



reaches the Hver except the small flow through the hepatic arteries, 

 the blood in the inferior caval vein is now nearly devoid of oxygen. 

 The sudden expansion of the lungs with the first breath results 

 in an immediate expansion of the pulmonary arteries and a simul- 

 taneous active contraction of the walls of the ductus arteriosus 

 forces all the blood from the right atrium to flow through these. 

 This contraction of the ductus arteriosus is maintained until the 

 lumen is permanently obliterated and the vessel remains only as a 

 solid cord, the arterial ligament of the adult. 



The increased stream of blood through the lungs returns through 

 the pulmonary veins to the left atrium, producing altered pressure- 

 relations there such that the flaps at each side of the foramen ovale 

 are pressed into contact. Thus an almost immediate functional 

 closure of the foramen takes place. Later, the flaps fuse and the 

 position of the foramen is indicated only by a thin area, the fossa 

 ovalis, in the adult heart. Such closure of the foramen ovale 

 diverts all the blood entering the right atrium into the right 

 ventricle, whence it all is pumped to the lungs, as just indicated. 



Thus all the regions of mixture of oxygenated and unoxygenated 

 blood are closed off at birth and thereafter all blood in the right side 

 of the heart is unoxygenated and all blood in the left chambers of 

 the heart is oxygenated. Moreover, the oxygenated blood from 

 the left ventricle is distributed through the aorta and its branches 

 to all parts of the body without any dilution such as is brought 

 about by the ductus arteriosus in the foetus, and even the most 

 posterior parts receive blood with as much oxygen as in that to the 

 head. 



Although the placenta is lost, the basal part of the outgrowth 

 of the embryonic alimentary canal which produced it remains 

 and forms the urinary bladder; and the corresponding portions of 

 the umbilical arteries also remain to supply that organ, though 

 reduced in relative size and now carrying oxygenated blood. 



The Lymphatic System 



The lymphatic system, both in its functional relation and in 



origin, is an appendage of the venous portion of the vascular system. 



The system is an important one, of which, unfortunately, little may 



be seen by ordinary dissection, the structures which are revealed 



