Sauropsida: Class Aves 511 



through it during the downstroke of the wing. These flight-feathers, 

 called "remiges" (singular, "remex") are arranged in a single row 

 with their large quills strongly and deeply inserted into the skin of 

 those parts of the wing which correspond to the forearm and hand of 

 man (Fig. 398). Those attached to the three-fingered hand are called 

 primaries, and those between wrist and elbow are secondaries. The 

 number of each is constant in a given species of bird. Similar feathers 

 called rectrices (singular, rectrix), symmetrically arranged in pairs 

 (commonly six or seven pairs) at the tail-end of the bird, are used for 

 steering. 



Feathers are the most complex and highly specialized of all the 

 structures produced by the skin of vertebrates. 



Avian skin is devoid of glands except for the oil-secreting uro- 

 pygial gland situated on the dorsal surface of the short tail (uropyg- 

 ium). By means of the beak, the bird distributes the waterproofing 

 oil over the plumage. The gland is absent in ostriches, parrots, and 

 some other birds, and is especially well developed in aquatic birds. In 

 lack of glands, avian and reptilian skins are similar. Distal to the 

 feathered regions of the legs, the skin produces horny scales of reptilian 

 type. 



Skeleton 



The function of the bird's hand is to support the remiges. Since 

 these are arranged in a single longitudinal row, a pentadactyl hand 

 would supply a superfluity of parts. In the hand of a bird there are 

 three digits (Fig. 401), of which only the middle one is well developed, 

 the other two being weak vestiges. The three probably correspond to 

 the thumb and first two fingers of the human hand. The three meta- 

 carpals are more or less fused together and the more distal carpals are 

 fused to the proximal end of the fused metacarpals, the result being a 

 complex carpo-metacarpus (Fig. 401, Mc). Two proximal carpals 

 (C, C") remain free. These several modifications of the typical limb 

 serve to provide a simple and strong axis for support of a row of 

 remiges. 



The pectoral girdle (Fig. 401) consists of the usual dorsal scapula 

 and ventral coracoid and clavicle, but the ventral ends of the right 

 and left clavicles are solidly fused to form the characteristic furcula 

 ("wishbone"), which is more or less strongly attached to the sternum. 

 The coracoid also is attached to the sternum, so that the girdle is well 

 anchored. 



In "taking off" for flight, the bird leaps vigorously into the air. 

 The legs are strongly developed. Somewhat as in frogs and toads, the 

 leg is made more effective for leaping by modifications which give it 

 four long movable segments (Fig. 402) instead of the three of ordinary 



