BIRDS. 243 



communication hehoeen the pulmonary and systemic circula- 

 tions, as there is in Reptiles. 



The respiratory organs are in the form of spongy celhdar 

 lungs, lohich 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. 



All birds are oviparous, none bringing forth their young 

 alive, or being even ovo-viviparous. All birds are, lastly, pro- 

 vided with an epidermic covering, so modified as to constitute 

 luhat are known as feathers. 



The entire skeleton of the Birds is singularly compact, and 

 at the same time singularly light. The compactness is due 

 to the presence of an unusual amount of phosphate of lime ; 

 and the lightness, to the absence in many of the bones of 

 the ordinary marrow, and its replacement by air. 



As regards the vertebral column, Birds exhibit some very 

 interesting peculiarities. The cervical region of the spine is 

 unusually long and flexible, since the fore-limbs are useless 

 as organs of prehension — and all acts of grasping must be 

 exercised either by the beak or by the hind-feet, or by both 

 acting in conjunction. The number of vertebrse in the neck 

 varies from eight to twenty-three. The front faces of their 

 centra are cylindroidal (spheroidal in Penguins), convex from 

 above downvKirds, and concave from side to side, the posterior 

 faces being concave from above downwards and convex from 

 side to side. Hence in vertical section, the vertebrae appear 

 to be opisthoccelous, and in horizontcd section procmlous. This 

 structure of the cervical vertebrae is highly characteristic of 

 Birds. The dorsal vertebra vary from six to ten in number, 

 and of these the anterior four or five are generally anchy- 

 losed with one another, so as to give a base of resistance to 

 the wings. In the Cursorial Birds, however (such as the 

 Ostrich and Emeu), and in some others (such as the Pen- 

 guin), in which the power of flight is wanting, the dorsal 

 vertebrae are all more or less freely movable one upon an- 

 other. There are no lumbar vertebra?, but all the vertebrse 

 between the last dorsal and the first caudal (varying from 

 nine to twenty) are anchylosed together to form a bone 

 which is ordinarily known as the " sacrum." To this, in 



