534 EMBRYOLOGY OF THE CHICK 



in the human heart these perforations form an oval-shaped opening called 

 the foramen ovale, which may, in abnormal cases, remain open and thus 

 cause a constant intermingling of venous and arterial blood. Usually, 

 such persons do not live long, although there are notable exceptions. 

 This inter-auricular foramen closes at the time of hatching, so that the 

 blood from the right auricle can be sent to the lungs for aeration as soon 

 as these organs become functional at birth. 



The septa are sufficiently developed so that we may speak of four, 

 divisions or cavities in the heart. This makes a double circulation pos- 

 sible, namely, the systemic and the pulmonary (up to the time of hatch- 

 ing, the allantoic circulation takes the place of the pulmonary). 



By this time, then, the heart is fully formed. The sinus venosus has 

 been absorbed into the right auricle, of which it forms a part. The open 

 foramina allow blood to pass back and forth between the auricles. The 

 ventricular septum is more complete. The truncus arteriosus is divided 

 into two separate vessels, the pulmonary trunk arising from the right ven- 

 tricle, and the systemic trunk from the left ventricle. 



The aortic arches which are still present are the third, fourth, and 

 fifth, and small portions of the first and second. 



The systemic trunk from the left ventricle leads to the third and 

 fourth pairs of aortic arches, from which the head and fore-limbs are 

 supplied. 



The pulmonary trunk, arising from the right ventricle, leads to the 

 fifth pair of aortic arches, which are directly continuous with the dorsal 

 aorta. It is from these that the small pulmonary arteries arise. 



It will be remembered that as the lungs are not yet functional, there 

 is little use for these vessels until later. An omphalomesenteric artery 

 carries blood to the yolk-sac and a large allantoic artery passes from the 

 aorta to the allantois. 



The venous system consists of the right and left anterior venae cavae, 

 and the posterior vena cava. The former drain the head and fore-limbs, 

 and the latter the posterior portions of the body, the limbs, and the kid- 

 neys. 



Before reaching the heart, the posterior vena cava is joined by the 

 ductus venosus, through which blood is returned from the yolk-sac, allan- 

 tois, and embryonic alimentary canal, by the omphalomesenteric, allan- 

 toic, and mesenteric veins respectively. 



All three venae cavae open into the right auricle of the heart, but 

 due to the position and direction of the opening, and to a valve, tne t)lood 

 from the posterior vena cava is directed through the foramen ovale into 

 the left auricle, while the blood from the right and left venae cavae (an- 

 terior) remains in the right auricle. 



As the auricles now contract, the blood which has come from the 

 posterior vena cava is forced into the left ventricle and passes out through 

 the systemic trunk through the third and fourth pairs of aortic arches 



