CHARACTERS OF AVES. 423 



DIVISION II. SAUROPSIDA. 



CHAPTER LXVI. 



CLASS IV. AVES. 



THE fourth class of the Vertebrata is that of Aves, or Birds. 

 The Birds may be shortly defined as being " oviparous Verte- 

 brates 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 embryo possesses an amnion and allantois, and branchiae 

 or gills are never developed at any time of life upon the visceral 

 arches. The skull articulates with the vertebral column by a 

 single occipital condyle. Each half or ramus of the lower jaw 

 consists of a number of pieces, which are separate from one 

 another in the embryo ; and the jaw is united with the skull, 

 not directly, but 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 metacarpal bones 

 are anchylosed together. In all living Birds the fore-limbs are 

 useless as regards prehension, 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 

 coalescing with the tibia, and the distal portion of the tarsus 

 being anchylosed with the metatarsus to constitute a single bone 

 known as the " tarso-metatarsus." 



The heart consists of four chambers, two auricles, and two 

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

 heart completely separated from one another, but there is no 

 communication between the pulmonary and systemic circula- 

 tions, as there is in Reptiles. There is only one aortic arch, 

 the right. The blood is hot, having an average temperature of 

 as much as 103 to 104. The blood-corpuscles are oval and 

 nucleated. 



The respiratory organs are in the form of spongy cellular 

 lungs, which are not freely suspended in pleural sacs; and 

 the bronchi open on their surface into a number of air-sacs, 

 placed in different parts of the body. 



