242 



CHAPTER XXXVL 



BIRDS. 



The fourtli class of the Verkhrata is that of Aves, or Birds. 

 The Birds may be shortly defined as being " oviparous 

 Vertebrates with warm blood, a double circulation, and a 

 covering of feathers " (Owen). More minutely, however, 

 the Birds are defined by the possession of the following 

 characters : — 



The skull articulates with the vertcljral column hy a single 

 occipital condyle. The form of the vertebral centra varies; 

 hut they are in no case am^jhicmlous, except in the extinct 

 Ichthyornis. Each half or ramus of the lower jaw consists 

 of a number of p)ieees, which are separate from one another in 

 the embryo ; and the jaw is united with the skull, not directly, 

 hut by the intervention of a quadrate bone (as in the Reptiles). 

 The fore-limb in no existing birds possesses more than three 

 fingers or digits, and the metaearpcd bones are anehyloscd to- 

 gether. In all living Birds the fore-limbs are useless as re- 

 gards pirehension, and in most they are organs of flight. The 

 hind -limbs in all Birds have the ankle-joint placed in the 

 middle of the tarsus, the proximal portion of the tarsus coalesc- 

 ing with the tibia, and the distal ijortion of the tarsus being 

 anehyloscd ivith the metatarsus to cooistitute a single bone known 

 as the " tarso-metatarsus." 



The heart consists of four chambers, two auricles and tivo 

 ventricles; and not only arc the right and left sides of the 

 heart completely separated from one another, but there is no 



