Characteristics of the Vertehrata. xlix 



of the lower jaw is observable, so, that in the organism of a single 

 Monodelphous animal are combined peculiarities of both the other 

 Sub-classes. Marsupial bones are never present in Monodeljj/iia. 



Class, Aves. 



Air-breathing warm-blooded Vertebrata, which have epidermal 

 appendages of the structure of feathers, and which are always 

 oviparous. Their anterior pair of limbs have the shape of wings, 

 which are formed thus : the two digits of the ulnar side are aborted 

 together with their metacarpals ; the remaining three metacarpals, 

 and the os magnum of the carpus, are fused into a single bone, 

 upon which three digits are carried, and which abuts proximally 

 upon two free carpal bones. Of these digits the middle one, 

 corresponding to the index finger, and the carpo-metacarpal bone, 

 carry the ' primary ' quill feathers, whilst the ulna carries the 

 ^ secondaries,^ and the humerus the ' scapularies ' s. ' parapterum ' 

 of pterylography. Most of the peculiarities which distinguish the 

 Avian from the Reptilian organism, are to be correlated more or 

 less directly with their power of flight. The necessity for the 

 possession of the power of exerting great muscular force entails 

 the possession of warm blood; and the immobility of the dorsal, 

 the pliability of the cervical, and the great extent of the sacral 

 vertebral regions, are nearly as directly connected with the function 

 of flight jjs the great development of the sternum, whence the 

 muscles of flight take origin, or the conformation of the limbs upon 

 which they act. The warm-bloodedness or ' homoeothermal ' character 

 of Birds, might appear to connect them more closely with Mam- 

 mals than with Reptiles ; but it will be found to be correlated with 

 but few distinctively IVIammalian characters, beyond those which 

 may be expressed by saying that in both these homoeothermal 

 classes, the venous and arterial systems are prevented from directly 

 intermingling their blood by the existence of a quadrilocular heart, 

 and of a single systemic aorta ; whilst the brain holds a more 

 favourable relation quantitatively to the body and to the spinal 

 cord ; and the spinal cord again is, with perhaps a few exceptions, 

 larger relatively to the body than is observed to be the ease in cold- 

 blooded Vertehrata of any Class. On the other hand, the totality 

 of the Avian organization, with the exception of the epidermal 



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