380 



ZOOLOGY 



are called distal barbules, and are provided with hooks, whereas 

 the organs facing the base of the feather are called proximal 

 barbules, and bear flanges. The hooks of the one l)arl3 inter- 

 lock with tlie flanges of the next following barb, and thus all 

 the Ijarbs are held together in one firm web. 



Skeleton. — The bones of birds are exceedinglj^ light, manj^ 

 of th(>m being hollowed out and filled with sacs connected with 

 the lungs and containing heated air. This hot air greatly 



Fi(_;. 3.51. — Rtructure of a feather. Two l>arb3 are represented cut acrc^s. 

 Each bears to the left three "di.stal l:)ar}:)ules " armed with hooks which 

 engage in the ffanges of the "proximal barbules" lying to the right of the 

 neighboring barb. After Pycraft. 



increases the Ijuoyancjr of the bird in the air. The axial 

 skeleton differs from that of most other vertel)rates in the 

 great length of the neck region, the compactness and rigidity 

 of the trunk, and the shortness of the tail (Fig. 352). The 

 latter ends in a scjuare bone which consists of a lot of rudimen- 

 tary vertcbrtB fused together. This piece corresponds to most 

 of the long tails of reptiles, and in some fossil birds ( Archeopte- 

 rix, see also Fig. 388) the vertebrce of the tip of the tail are still 

 distinct. The skull is very compact and light, and the limits of 

 the different bones of which it is com])osed are hard to make out 

 except in young specimens (Fig. 353). The lower jaw is sup- 

 ported from the skull, as in the lower vertebrates, by a special 



