574 MANUAL OF ZOOLOGY. 



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 "second- 

 aries," 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 constitute the "tail-coverts." Generally there are 

 ten or twelve "rectrices ; " but there may be as many as twenty-four (as in 

 the Pelican), or rarely more ; and they do not carry "accessory plumules." 

 In addition to the "clothing-feathers " and the quill-feathers of the wings 

 and tail, the body is protected by a more or less abundantly developed 

 coating of " down-feathers" (" plumulse "), in which the barbules are not 

 hooked, and the barbs are therefore free. In some cases there is no shaft 

 to the down-feathers, and the barbs are attached in a tuft to the end of the 

 quill. In other cases, the feathers closely approximate to hairs in form, 

 being very long, slender, and flexible. These "filoplumse" consist of a 

 delicate shaft, either destitute of vanes, or carrying a few barbs at the 

 extremity. 



Though apparently completely covered with feathers, these appendages 

 are really almost always confined to certain special tracts ("pterylse ") in 

 the body of a bird, the intervening spaces ("apteria") being, with few 

 exceptions, naked. These feathered and unfeathered regions are definite 

 in form, size, and arrangement in many great groups of birds, and can thus 

 be used as an important aid to classification. 



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 cervical 

 vertebrae vary in number from eight to twenty-three. The front 

 faces of their centra are cylindroidal (spheroidal in Penguins), 

 convex from above downwards, and concave from side to side, the 

 posterior faces being saddle-shaped, concave from above down- 

 wards and convex from side to side. Hence in vertical sec- 

 tion, the vertebrae appear to be opisthocxlous, and in horizontal 

 section- proccelous. This structure of the cervical vertebrae is 



