480 



DI VISION II. SA UROPSIDA . 



CHAPTER LXVI. 



CLASS IV. A VES. 



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 arnnion 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. The form of the vertebral centra 

 varies ; but they are in no case amphiccelous, except in 

 the remarkable extinct form described under the name of 

 Ichthyornis. 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 circulations, 

 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. 



