AVKS BIRDS. G27 



narrower than the root of the aorta on the right side. On the 

 right side, in fact, the middle arch now becomes of great import- 

 ance, and really constitutes the commencement of the descending 

 aorta, receiving the other communications as subordinate parts. 



The bird having escaped from the egg, and having breathed for 

 some time, all the blood from the right ventricle passes into the 

 lungs, the ductus arteriosi become totally imperforate, and two dis- 

 tinct circulations are thus established; one proceeding from the right 

 side of the heart through the lungs into the left side of the heart, 

 the other from the left side of the heart through the system into 

 the left side of the heart. 



We see, therefore, that of the five pairs of vascular branchial 

 arches which at first by their union formed the aorta as in Fishes, 

 those of the first pair on both sides and of the fifth on the left side 

 speedily disappear. The third on each side become the brachio- 

 cephalic trunks, the fourth of the right side becomes the descending 

 aorta, while the fifth of the right side and the fourth of the left side 

 are converted into the pulmonary arteries. The very short trunk 

 common to the two pulmonary arteries, as well as the equally 

 short trunk of the aorta, properly so called, are produced by the 

 transformation of the single cavity of the original " bulbus arte- 

 riosus" into two distinct canals, and thus this wonderful meta- 

 morphosis is accomplished. 



(703.) About the hundred and twentieth hour from the com- 

 mencement of incubation, the vascular layer of the blastoderm has 

 spread extensively over the yolk (Jig. 289, b ) ; and, as the vessels 

 formed by it become perfected, they are found to converge to the 



Fig. 289. 





