CHARACTEFS OF AVES. 483 



may, however, be as large as the original feather, or it may be 

 reduced to nothing more than a tuft of down. 



The feathers vary in different parts of the bird, and are 

 generally divided into those which cover the body " clothing 

 feathers" and those which occur in the wings and tail " quill- 

 feathers." As regards the great quill-feathers of the wings, the 

 longest are those which arise from the bones of the hand, and 

 they are called the " primaries." Those which arise from the 

 distal end of the fore-arm (radius and ulna) are termed the 

 " secondaries," and those which are attached to the proximal 

 end of the fore-arm are the " tertiaries." The feathers which 

 lie over the humerus and scapula are the " scapulars." The 

 rudimentary "thumb" also carries some quills, which form 

 what is known as the " alula/' or " bastard-wing." The smaller 

 feathers, which cover the bases of the quill-feathers above and 

 below, are the "wing-coverts" "greater," "lesser," and 

 "under." The great quill-feathers of the tail (" rectrices ") 

 form a kind of fan, of great use in steering the bird in flight; 

 and their bases are covered by a series of feathers which con- 

 stitute the " tail-coverts." 



The entire skeleton of the Birds is singularly compact, and at 

 the same time singularly light. The compactness is due to the 

 presence of an unusual amount of phosphate of lime ; and the 

 lightness, to the absence in many of the bones of the ordinary 

 marrow, and its replacement by air. 



As regards the vertebral column, birds exhibit some very in- 

 teresting peculiarities. The cervical region of the spine is 

 unusually long and flexible, since the fore-limbs are useless as 

 organs of prehension ; and all acts of grasping must be exer- 

 cised either by the beak or by the hind-feet, or by both acting 

 in conjunction. In all birds alike, the neck is sufficiently long 

 and flexible to allow of the application of the beak to an oil- 

 gland placed at the base of the tail, this act being necessary 

 for the due performance of the operation of " preening " that 

 is, of lubricating and cleaning the plumage. The number of 

 vertebrae in the neck varies from nine to twenty-four, and their 

 structure is always such as to allow of considerable freedom of 

 motion one upon the other. The dorsal vertebras vary from 

 six to ten in number, and of these the anterior four or five are 

 generally anchylosed with one another, so as to give a base of 

 resistance to the wings. In the Cursorial Birds, however (such 

 as the Ostrich and Emeu), and in some others (such as the 

 Penguin), in which the power of flight is wanting, the dorsal 

 vertebrae are all more or less freely movable one upon another. 

 There are no lumbar vertebrae, but all the vertebrae between 



