IX.] SUMMAKY. 297 



'ductus arteriosi.' The one on the right side is short; 

 that on the left side is much longer and narrower. 



When respiration commences the blood ceases to 

 pass through these canals, which either remain as mere 

 ligaments or else become absorbed altogether. By this 

 means, the foramen ovale becoming at the same time 

 closed, a complete double circulation is established. All 

 the blood from the right ventricle passes into the lungs, 

 and all that from the left ventricle into the body at 

 large. 



Two other changes take place about the same time 

 in the aortic branches. That portion of the right fourth 

 or aortic arch which lies between the origin of the right 

 subclavian and the common carotid becomes shortened, 

 and is finally swallowed up in such a fashion that the 

 right subclavian (Fig. 94, R. sc.) comes off from the 

 right common carotid, a very short trunk being formed 

 by the union of the two to serve as the right innomi- 

 nate artery. 



At the same time, corresponding to the increase in 

 the length of the neck, the common carotids are very 

 greatly lengthened. They lie close together in the 

 neck, and in many birds actually unite to form a com- 

 mon trunk. 



It will of course be understood that with the dis- 

 appearance of the allantois and the absorption of the 

 yolk, the allantoic and vitelline arteries also disappear. 



It may perhaps be of advantage to the reader if we 

 here briefly summarize the condition of the circulation 

 at its four most important epochs; viz. on the third 

 day, on the fifth day, during the later days of incu- 

 bation before respiration by the lungs has commenced, 



