110 



SUBKINGDOM VERTEBKATA. 



Fig. 183. 



CLASS II. AVES. 



General Characteristics. Birds are marked by being 

 clothed with feathers. The modifications in the skeleton of 

 a mammal necessary to adapt it for flight are very few, and 

 are mainly those pertaining to the fore limbs and the breast- 

 bone. The latter (g) is greatly enlarged to furnish support 

 to the powerful muscles* which move the wings; and the 



former have the bones below 

 the elbow (?) more or less 

 consolidated to give them a 

 firm stroke against the air. 

 The hand contains, in most 

 species, three fingers. The 

 clavicles (j) are generally 

 united at one end, forming 

 the "wish-bone." To 

 strengthen the body the lum- 

 bar and sacral vertebrae are 

 joined to the hip-bones (r), 

 the dorsal are more or less 

 united, and the ribs are ossi- 

 fied throughout, with each a 

 process (/) lapping upon the 

 adjoining rib. The inner toe or thumb has two phalangeal 

 bones ; the next, three ; the middle, four ; and the last, five. 

 The number of cervical vertebra) (b) varies from nine to 

 twenty-three ; and the upper one articulates with the skull 

 by only one process, thus aiding in that variety of motion so 

 essential in the act of " preening." 



The bones are light. This is owing to their being com- 

 posed largely of phosphate of lime, and the marrow in many 

 of them being replaced by air. Singularly, at one stage they 

 are solid, like those of all Vertebrates, but the bony tissue is 

 afterward absorbed. "The thinnest- walled and widest air- 



Skdeton of a Bird. 



The pectoral muscles form the great part of the so-called " breast " of a fowl. 



