748 AYES. 



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. 



(2130.) 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 commencement of the aorta 

 towards the head. Thus, one part of the primitive root of the aorta 

 becomes the trunk of the carotid artery. 



(2131.) 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. 



(2132.) 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 respective 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. 



(2133.) 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 narrower than the root of the 

 aorta on the right side. On the right side, in fact, the middle arch now 

 becomes of great importance, and really constitutes the commencement 

 of the descending aorta, receiving the other communications as sub- 

 ordinate parts. 



(2134.) 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 distinct 

 circulations are thus established one proceeding from the right side of 



