306 THE CHICK. 



arches, and divides the truncus arteriosus in such manner that 

 the right division of the truncus arteriosus, and consequently 

 the right ventricle, from which this division arises, sends all its 

 blood into the fifth pair of aortic arches ; while the left ventricle 

 and left division of the truncus arteriosus conduct blood to the 

 third and fourth pairs of arches. It follows from this that the 

 supply of blood to the head and anterior part of the body is 

 derived from the left ventricle ; while the right ventricle supplies 

 the whole of the body behind the heart, as well as the yolk-sac 

 and allantois. 



About the seventh day the two divisions of the truncus 

 arteriosus separate completely from each other at their bases ; 

 the right branch, or pulmonary trunk, remaining in connection 

 with the right ventricle and the fifth pair of aortic arches ; and 

 the left branch, or systemic trunk, with the left ventricle and 

 the third and fourth pairs of aortic arches. 



The part of the aorta connecting the dorsal ends of the third 

 and fourth pairs of aortic arches (Fig. 128) becomes very slender, 

 and is finally obliterated altogether, while the branch of the 

 truncus arteriosus from which the third aortic arch arises elon- 

 gates very considerably, and carries this arch forwards some 

 distance in front of the next, or fourth aortic arch. The subclavian 

 arteries, supplying the fore-limbs, arise from the ventral ends of 

 the third aortic arches, during the third day ; they grow back- 

 wards, lying ventral to the other vessels, and reach the limbs 

 during the fourth or fifth day. 



The fourth pair of aortic arches is, from the fifth day 

 onwards, much the largest of the three persistent pairs. The 

 arches of the two sides of the body are at first of equal size ; but 

 the arch of the right side soon becomes much larger than that 

 of the left side ; and the latter ultimately becomes obliterated 

 along the greater part of its length, while the arch of the right 

 side persists as the arch of the aorta in the adult bird. 



The pulmonary arteries appear, in the walls of the lungs, 

 about the middle of the third day, before the two hinder pairs 

 of aortic arches are formed. On the appearance of the fifth pair 

 of aortic arches the pulmonary arteries (Fig. 128, AP) become 

 connected with their ventral ends. Each fifth aortic arch thus 

 consists of two parts : a proximal part running from the truncus 

 arteriosus, i.e. from the right ventricle, to the lung ; and a distal 



