66 COMPARISONS OF STRUCTURE IN ANIMALS. 



fingers or their representatives, one composed 

 of two joints, the other a mere rudiment. The 

 shoulders are kept apart by the merry-thought 

 or furcula, and the narrow blade-bones are 

 fiu-ther strengthened by two additional clavi- 

 ciilar bones, which, joining one on each side 

 Avith the breast-bone, oppose the inward strain 

 of the powerful muscles of the breast. The 

 bones of the wing of a bird constitute the 

 framework, upon which the large feathers 

 essential to flight are arranged and fixed 

 firmly. Those on the fingers and metacarpus 

 are termed primaries, and much of the power 

 and character of the flight of the species 

 depends on their form and elasticity. Those 

 fixed on the fore-arm are termed secondaries, 

 and those on the upper-arm are termed scapu- 

 laries; but in many birds long slender feathers 

 proceed from the elboAv-joint, and are tei-med 

 tertiaries ; they are very observable in plovers, 

 lapwings, curlews, etc. The thumb gives 

 rise to short stiff" feathers, constituting the 

 winglet, or spurious wing. The extent of the 

 wing thus produced,- its general form, and the 

 rigidity of the feathers, greatly influence the 

 rapidity and style of progress in the air; and 

 in some birds — as the ostrich, the emeu, the 



