Class 5. Aves. 435 



knobs, plates, splints. The spur of the Oock and other male 

 gallinaceous Birds, which is provided with an internal ossification 

 firmly attached to the metatarsus, is a peculiar, large, conical 

 scale ; this spur is present in the Hen also, but usually as a simple, 

 wart-like scale.* 1 a w s are present on the toes of the hind foot ; 

 in perching forms they are long, curved, and pointed; in ground 

 Birds, shorter and thicker. They are often entirely absent from the 

 fore limbs; but there is frequently a small, often rudimentary, 

 ■claw on the poUex, and not seldom there is another such claw on 

 the second digit, but this is generally rudimentary ; it may sometimes 

 be present in the young one, and lost later j in the Ostriches of 

 Africa both claws are of fair size and well developed. In all living 

 forms the claw of the third finger is wanting; but in Archaeopteryx 

 well-developed claws are present on all three digits, a fact which 

 ■can be determined with certainty from the form of the last finger 

 (Fig. 378). The edges of the jaw and the adjacent parts of the head, 

 the beak, are usually covered by a thick, hard mass of horn, often 

 with sharp edges ; occasionally this is partly or entirely replaced by 

 a thinner and softer horny sheath. An ecdysis of the whole skin 

 like that of many Eeptiles, does not occur in Birds; the stratum 

 •corneum is thrown off in small portions. 



Birds usually possess only one pair of integumentary glands, the 

 large, round uropygial glands, which are situated dorsally on 

 the short tail ; they lie close together, and their apertures are near 

 ■each other, generally on a small papilla. Each gland consists of 

 numerous tubes, opening into a large central cavity, which is 

 continued into the duct; in some each gland has several ducts. 

 They secrete a lubricating fiuid, which the Birds remove with their 

 bills for preening their feathers ; they are largest in aquatic species, 

 and are absent from the Ostriches, some Parrots, and a few others. 



The skeleton. The vertebral column may be divided into 

 regions similar to those of Reptiles. The cervical vertebrse 

 are numerous ; there may be as many as twenty, very loosely articu- 

 lated with each other. The first and second vertebree, as in tbe 

 Eeptilia, are developed as atlas and axis (the centrum of the atlas 

 is fused with that of the axis, etc.). Further, the articular surfaces 

 of the cervical vertebra are saddle-shaped (each centrum is, 

 anteriorly, concave from right to left, convex dorsoventrally, and, 

 posteriorly, convex from right to left, concave dorsoventrally) ; in 

 ArohfBopteryx and some of the Odontornithes {Ichthyornis) the sur- 

 faces of the centra were fiat or feebly biconcave. For the cervical 

 ribs see below. The thoracic vertebrae, in contradistinctioii 

 to the cervicals, are somewhat few in number, and but slightly 



* On the fore limbs too, similar spurs may also occur, but these must not be 

 ■confused with the claws mentioned above. 



F F 2 



