284 THE STRUCTURE AND LIFE OF BIRDS chap. 



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 



Fig. 71.— Diagrams— after Boas— illustrating (a) lizard's heart ; (6) bird's heart ; 

 (c) human heart, viewed from the front ; 6, 5, 4, the 6th, 5th, 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 



