282 THE STRUCTURE AND LIFE OF BIRDS cnap. 
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. 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 
Fic. 69.—Hyoid bone of Crow. the second, and so on. There 
wysecond arch 5 a,third arch. 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. 71). On 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. 
