. The structure and life of birds . rst, second, and fifth pairshave been obliterated. with them, and here we have the final arrangement,the artery from the right side of the heart leading tothese arches, and, so, onward to the lungs. Mean-while the third pair of arches has been growing weaker,and the fourth stronger. Finally, only one of the fourthpair is left, that on the right side ; it is the great aorta,which may be seen curving to the right, one of thethree single survivors out of five pairs, the other twobeing the pulmonary arteries, which lead to the leftand right lungs and represent t
. The structure and life of birds . rst, second, and fifth pairshave been obliterated. with them, and here we have the final arrangement,the artery from the right side of the heart leading tothese arches, and, so, onward to the lungs. Mean-while the third pair of arches has been growing weaker,and the fourth stronger. Finally, only one of the fourthpair is left, that on the right side ; it is the great aorta,which may be seen curving to the right, one of thethree single survivors out of five pairs, the other twobeing the pulmonary arteries, which lead to the leftand right lungs and represent the sixth pair. Occa- 284 THE STRUCTURE AND LIFE OF BIRDS chap. sionally the left arch, opposite to the aorta, may beseen in a rudimentary form. I have once seen it in achicken. In mammals it is the left arch, instead of theright, of the fourth pair that survives, and forms amost important distinction between them and generally have only the fourth pair of archessurviving, but in many lizards both the third and fourth. Fig. 71.—Diagrams—after Boas—illustrating (a) lizards heart; (i) birds heart ;(c) human heart, viewed from the front; 6, 5, 4, the 6th, 5th, and 4th pairs of aorticarches. are found. For a time the bird is, judged by hisheart, a reptile. The crocodiles heart, which in severalpoints comes near to that of birds, may be called anoble failure. It has four chambers instead of, likemost reptilian hearts, only three. But it has two aortasinstead of one. Of these, one springs from the leftventricle and carries j^ure blood ; the other from the viii THE BIRD WITHIN THE EGG 285 right ventricle and carries impure. Outside the heartthe two arteries cross and there is a small passagefrom one to the other, so that the pure and impureblood are mixed. The fourth pair of arches havebecome separate except at this one point; but whatshould have followed, has not—viz. the reduction ofthe right aorta to a rudimentary state. Otherwisethe crocodile might have becom
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectbirds, bookyear1895
