282 THE STRUCTURE AND LIFE OF BIRDS chap. 



is porous, and allows of the passage of gases out 

 and in. 



All the pairs of arches except three, the first, second, 

 and third, become obliterated in the mature bird. The 

 first pair become the lower jaw : branches sent* out 

 from them form the upper 

 jaw. The hyoid bone, which 

 lies at the bottom of the 

 mouth and helps to support 

 the tongue, represents the 

 second and third pairs. The 

 first cleft, the only one which 

 does not get obliterated, be- 

 comes connected with the or- 

 gan of hearing, and forms the 

 eustachian tubes. 1 The bran- 

 ches of the aortic artery form 

 pairs of arches corresponding 

 to the folds visible on the 

 surface. The first pair of 

 aortic arches lies within the 

 first fold, the second pair in 

 the second, and so on. There 

 are five folds and five pairs of 

 aortic arches within them. At 

 a very early stage there is another pair between 

 No. 4 and that just spoken of as No. 5, which must, 

 therefore, rank as No. 6 (Fig. 70- ° n the fourth 

 day the first two pairs also disappear, leaving 

 only three, and we must carefully trace the history 

 of these three and their connection with the heart. 

 1 See p. 134- 



Fig. 69.— Hyoid bone of Crow. 

 2, second arch ; 3, third arch. 

 The jaws are the first. 



