14 ANATOMY OF VEUTEBKATES. 



CHAPTEE XIV. 



OSSEOUS SYSTEM OF AVES. 



§ 124. General Characters. — The skeleton of Birds is remark- 

 al)le for the rapidity of its ossification and the light and elegant 

 luechanism displayed in the adaptation of its several parts. The 

 osseous substance is compact, and exhibits more of the laminated 

 and less of the fibrous disposition than in the other vertebrate 

 classes. This is more especially the case in those parts of the 

 skeleton which are permeated by the air. The bones which pre- 

 sent this singular modification have a greater proportion of the 

 phosphate of lime in their composition than is found in the osseous 

 system of the mammalia, and they are whiter than the bones of 

 any other animal. In the bones where the medulla is not displaced 

 by the extension of the air-cells into their interior, the colour is of 

 a duller white. In the Silk- or ' black-boned ' fowl of the Tropics 

 ( Gallus Morio, Temminck), the periosteal covering of the bones is 

 of a dark colour ; but this is a peculiarity of the cellular rather 

 than of the osseous texture, which does not difier in colour from 

 that of other birds ; indeed the thin aponeurosis covering the 

 lateral tendons of the gizzard of the Silk-fowl has the same dark 

 hue as the membrane which invests the bones. 



§ 125. Dorsal Vertehrce. — The modifications of the common 

 vertebrate type of skeleton required by the exigencies of the pre- 

 sent class are extreme. Anchylosis so fetters the vertebral column 

 that from no part can a single segment with all the elements be 

 detached without using the saw. The skull includes four, the 

 sacrum a greater number, of vertebras, of more or less of which 

 the haimal portions alone retain freedom. The remaining segments 

 may be classified as ^ cervical,' * dorsal,' and ' caudal ' : in the first 

 and last the pleurapophysis, if present, is confluent with the 

 neural arch : in the dorsal series, the pleur- and hajm-apophyses 

 are flexibly articulated, but the hasmal spines are connate, and 

 represented by a single bony plate. 



