626 



AVES BIRDS. 



and on the seventh day of incubation is no longer recognisable ; 

 whilst in the mean time the current of blood from the right ven- 

 tricle is directed in such a manner as to pass in front of this arch, 

 and enters the posterior arch of the right side, and the last but 

 one on the left. 



As, moreover, the two arches, that were formerly the most 

 anterior, have become obliterated, while the third and fourth, on 

 the contrary, are increased in size, the blood, passing backwards 

 through these arches into the roots of the aorta, enters also the 

 carotid artery, which now resembles a prolongation of the com- 

 mencement of the aorta towards the head. Thus, one part of the 

 primitive root of the aorta becomes the trunk of the carotid artery. 



There exist, consequently? on the eighth day three vascular 

 arches on the right side, and only two on the left ; and these five 

 arches are derived from the heart, as are also two small vascular 

 trunks now quite distinct, which have been formed from the bulb. 



The anterior arch of both sides and the middle arch of the right 

 side proceed from the left ventricle ; the posterior arches issue 

 from the right : but all of them as yet unite to form the two roots 

 of the aorta, which are still of pretty equal size, and each root 

 gives off a carotid artery. At the point where the anterior arches 

 join the roots of the aorta, they are now seen to give off newly- 

 formed trunks, which go to the anterior extremity of their respec- 

 tive sides ; and as these limbs and the head increase in size, and 

 require more blood, the anterior arch propels a greater proportion 

 of blood in that direction, and insensibly less and less into the 

 aorta. The consequence is, that the anterior arch becomes more 

 and more decidedly the brachio-cephalic trunk ; and in short, on the 

 thirteenth day, it only communicates with the dorsal aorta by a 

 small vessel, and ultimately becomes quite detached, forming the 

 arteria innominata of the corresponding side. 



Meanwhile the posterior arches on both sides send out branches 

 destined to the contiguous lungs. On the eighth day these vessels 

 are still very small, and difficult to find ; but they soon grow larger, 

 and, during the last half of the period of incubation, they show 

 themselves as the immediate continuations of the arches from which 

 they are derived ; their junctions with the aorta becoming more 

 and more imperfect, and constituting the two ductus arteriosi. 

 These canals are of very unequal size ; that of the right side is 

 much shorter than that of the left, which is now the only remnant 

 of the original root of the aorta on that side, and considerably 



