834 DEVELOPMENT. [CH. LVIII. 



The replacement of the circulation in the yolk-sac, or umbilical 

 vesicle, by the allantoic or placental circulation is illustrated by 

 the two preceding diagrams (figs. 652, 653). 



As the body develops, new arteries and new veins form, and the 

 heart becomes more complicated. The ventricle is divided into 

 two (right and left) by a septum ; the bulb divides into two, and 

 one division leads from the right ventricle to the pulmonary 

 artery ; the other leads as the main aorta from the left. The 

 auricles are also divided into two (right and left), but the com- 

 plete separation of the two auricles does not take place till after 

 birth. Before birth, as we shall see when describing the later 

 foetal circulation, some of the blood which enters the right auricle 

 passes into the left auricle by a wide opening called the foramen 

 ovale. 



The pulmonary artery leads direct into the descending aorta; 

 the branches to the lungs are small and unimportant : it is not till 

 the child is born, and begins to use its lungs, that the arteries to 

 them assume importance ; then also the communication with the 

 aorta is closed, and remains as a cord, called the ductus arteriosus. 



These changes will be grasped better if we look at the two next 

 diagrams (figs. 654, 655). 



The heart (fig. 654) is in a rather more advanced stage than in 

 fig. 651 ; it is beginning to get a twist which is bringing the ven- 

 tricle, now increasing in size, into its subsequent position ; but it 

 is seen that instead of two simple arches uniting to form a dorsal 

 aorta, there are now five. These correspond to the gill arteries of 

 fishes, but in mammals never break up into capillaries, as in a 

 fish's gills. They, however, run in the visceral arches, between the 

 visceral clefts. In amphibia, three pairs persist through life. 



In reptiles the fourth pair remains throughout life as the per- 

 manent right and left aorta ; in birds the right one remains as the 

 permanent aorta, curving over the right bronchus instead of the 

 left as in mammals. 



In mammals the left fourth aortic arch develops into the per- 

 manent aorta, the right one remaining as the subclavian artery 

 of that side. Thus the subclavian artery on the right side cor- 

 responds to the aortic arch on the left, and this hornology is 

 further confirmed by the fact that the recurrent laryngeal nerve 

 hooks under the subclavian on the right side, and the aortic arch 

 on the left. 



The fifth arch disappears on the right side, but on the left forms 

 the pulmonaiy artery. The distal end of this arch originally 

 opens into the descending aorta, and this communication (which is 

 permanent throughout life in many reptiles on both sides of the 



