. The structure and life of birds . three, the first, second,and third, become obliterated in the mature bird. Thefirst pair become the lower jaw: branches sent* outfrom them form the upperjaw. The hyoid bone, whichlies at the bottom of themouth and helps to supportthe tongue, represents thesecond and third pairs. Thefirst cleft, the only one whichdoes not get obliterated, be-comes connected with the or-gan of hearing, and forms theeustachian The bran-ches of the aortic artery formpairs of arches correspondingto the folds visible on thesurface. The first pair ofaortic arches lies withi
. The structure and life of birds . three, the first, second,and third, become obliterated in the mature bird. Thefirst pair become the lower jaw: branches sent* outfrom them form the upperjaw. The hyoid bone, whichlies at the bottom of themouth and helps to supportthe tongue, represents thesecond and third pairs. Thefirst cleft, the only one whichdoes not get obliterated, be-comes connected with the or-gan of hearing, and forms theeustachian The bran-ches of the aortic artery formpairs of arches correspondingto the folds visible on thesurface. The first pair ofaortic arches lies within thefirst fold, the second pair inthe second, and so on. Thereare five folds and five pairs ofaortic arches within them. Ata very early stage there is another pair betweenNo. 4 and that just spoken of as No. 5, which must,therefore, rank as No. 6 (Fig. 70- °n the fourthday the first two pairs also disappear, leavingonly three, and we must carefully trace the historyof these three and their connection with the See p. 134-. Fig. 69.—Hyoid bone of , second arch ; 3, third jaws are the first. VIII THE BIRD WITHIN THE EGG 283 By the end of the fifth day the ventricle of the hearthas divided into right and left chambers. On theseventh day the artery leading from it follows suit anddivides into right and left branches, which are quiteseparate. And of these two arteries, that from theright chamber connects with the sixth pair of arches,while that from the left connects with the thirdand fourth. Meanwhile the lungs have come intoexistence, being in their origin pouches formed inthe gullet. The sixth pair of arches open a connection
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