CIRCULATION AND RESPIRATION IN ANIMALS 171 



For our purpose it is sufficient to emphasize that in Man's 

 body one branchial arch continues to carry blood directly 

 from the heart to the dorsal aorta, while parts of another 

 deliver blood from the heart to the lungs and back again to 



D E F 



FIG. 95. Diagram to show the transformation of the six pairs of primitive gill slit 

 arteries (aortic arches) in the ascending series of Vertebrates. A, primitive condition, 

 embryonic; B, Fish; C, Amphibian (Frog); D, Reptile; E, Bird; F, Mammal, a, 

 dorsal aorta; b, ventral artery from heart; c, internal carotids; d, external carotids; 

 e, e', right and left aortic arches; /, pulmonary arteries; g, g', subclavian arteries to 

 fore limbs. 



the heart. Thus there is established a second current of blood 

 through the heart, which necessitates a median partition in 

 both the auricle and ventricle in order to keep the two cur- 

 rents separate. In this way a four-chambered heart arises 



