284 THE STRUCTURE AND LIFE OF BIRDS cua. 
sionally the left arch, opposite ‘to the aorta, may be 
seen in a rudimentary form. I have once seen it in a 
chicken. In mammals it is the left arch, instead of the 
right, of the fourth pair that survives, and forms a 
most important distinction between them and birds. 
Reptiles generally have only the fourth pair of arches 
surviving, but in many lizards both the third and fourth 
¢c 
Fic. 71.—Diagrams—after Boas—illustrating (a) lizard’s heart ; (4) bird’s heart ; 
(c) mas heart, viewed from the front ; 6, 5, 4, the 6th, sth, and 4th pairs of aortic 
arches. 
are found. For a time the bird is, judged by his 
heart, a reptile. The crocodile’s heart, which in several 
points comes near to that of birds, may be called a 
noble failure. It has four chambers instead of, like 
most reptilian hearts, only three. But it has two aortas 
instead of one. Of these, one springs from the left 
ventricle and carries pure blood ; the other from the 
