CIRCULATORY ORGANS. 283 
carrying blood from the ventral to the dorsal aortz, while the fifth, re- 
duced in size, perform a similar function in a few urodeles (fig. 280, C), 
but elsewhere they entirely disappear. The fourth arches show a dif- 
ferentiation between the two sides in many reptiles. That on the left 
side becomes separated from the rest of the ventral aorta (fig. 280, E, F) 
and has its own trunk connecting with the right side of the partially 
divided ventricle, and, as will be understood from the relations of the 
heart (p. 281), it may carry a mixture of arterial and venous blood. 
From the dorsal side, this blood of the left fourth arch is largely dis- 
tributed to the digestive tract, the cceliac axis arising from its radix, 
while the part connecting it with the dorsal aorta is reduced in size. 
The right arch and the carotids are connected with the left side of the 
Fic. 288.—Aortic arches of amniotes, after Hochstetter. A, Varanus; B, snake; C, 
alligator; D, bird; Z, mammal. 4, basilar artery; cc, common carotid; ch, ce, internal and 
external carotids; da, dorsal aorta; p, pulmonary; s, subclavian. 
heart and hence are purely arterial, the arch forming the main trunk 
connecting the heart with the dorsal aorta. In the birds (fig. 280, G) 
the radix of the left side of the adult disappears distal to the origin of 
the subclavian artery, so that this arch supplies only the fore limb of 
that side, while the right arch is purely aortic in character. In the 
mammals (fig. 280, H) these relations are exactly reversed, the right 
arch being subclavian, the left supplying the dorsal aorta and the 
subclavian of that side. 
With the development of lungs (dipnoi, tetrapoda) a pair of pul- 
monary arteries are developed from the sixth pair of arches on the 
ventral side of the pharynx. These grow back into the lungs, while the 
rest of the arch, dorsal to their origin, becomes reduced to a small vessel 
the ductus arteriosus (d. Botallii) in some urodeles, and persists 
occasionally vestigially in higher vertebrates. Elsewhere it entirely dis- 
appears. Inthe dipnoi and amphibia, where the ventricle remains 
