166 The Vertebrate Organ Systems 



With the loss of gill respiration, the need for these numerous 

 arches disappeared. In typical economical manner, the elements of the 

 arches were not completely discarded; rather they were converted to 

 new uses. In many amphibians, for example, the third arch is trans- 

 formed into the carotid artery for carrying blood forward to the head 

 region, and the first two arches are completely gone. In salamanders 

 the fourth and fifth arches form the systemic arches, that is, they join 

 after leaving the heart to form the dorsal aorta. Among the frogs and 

 toads, only the fourth arch forms the systemic arches. The sixth arch 

 becomes the pulmonary artery. 



In reptiles, the fifth arch disappears, and only the fourth remains 

 as the aortic arch. This loss of parts is carried to the extreme in birds 

 and mammals where but one-half of the fourth arch remains — the left 

 in mammals and the right in birds. The sixth remains as the pul- 

 monary artery in both, and the third continues to function as the 

 carotid. 



In the mammalian embryo, all these arches develop and gradually 

 transform into the adult condition. The ductus arteriosus of the fetus 

 actually is a remnant of the former connection of the sixth arch to the 

 aorta. 



The Pulmonary Circuit. — The chief large blood vessel of this 

 circuit is the pulmonary artery which arises from the right ventricle. 

 It bifurcates, shortly after leaving the heart, into the right and left 

 pulmonaries. Within the lung, these arteries break into numerous 

 arterioles and finally into the small capillaries which form part of the al- 

 veolar walls. These capillaries gradually anastomose into the venuoles 

 and veins which ultimately form the pulmonary veins which return the 

 blood to the left auricle. In the human being, there are four of these 

 veins, two from each lung. 



The Systemic Arterial System. — In the mammal (Fig. 48), the 

 principal artery of the systemic division is the aorta. It leaves the left 

 ventricle and turns sharply dorsad and to the left, making the large 

 aortic arch. At its base, the coronary arteries which supply the heart 

 muscle arise. From this arch, three principal vessels originate. The 

 first of these three is the innominate artery which soon branches into a 

 right subclavian going to the arm and shoulder and the right common 

 carotid supplying the head. The middle one of the three arteries is the 

 left common carotid which supplies the left side of the neck and head. 

 The last of these three branches is the left subclavian which goes to 



