VERTEBRATA AVES. 33 



corpuscles are oval and nucleated ; the embryo is provided 

 with an amnion and an allantois. 



34. Skeleton. In order to adapt birds for flight, the 

 skeleton is remarkable for its combined lightness and 

 strength. There is a larger proportion of phosphate of lime 

 present than in the bones of mammals ; and in adult birds, 

 many of the bones are filled with air instead of marrow. 



In full grown birds, the various bones that form the 

 skull, are amalgamated into a single piece. There is only 

 a single condyle on the occipital bone, as in the 

 reptiles. In young birds, each half of the lower jaw 

 consists of six pieces ; but these, in the mature animal, are 

 so amalgamated, that the lines of junction cannot be dis- 

 tinguished. The lower jaw is joined to the quadrate 

 bone, and not directly to the skull, as in the mammalia. 



The neck is long and flexible, and consists of from nine 

 to twenty-four, vertebrae. As the beak is the only organ 

 of prehension possessed by these animals, such a neck is 

 required to enable them to procure their food. It is also 

 long enough to reach a gland situated at the base of the 

 tail, where the oil with which they trim their feathers is 

 secreted. 



There are from six to ten dorsal vertebrae, several of 

 which are anchylosed together. By this arrangement, 

 that part of the vertebral column is made sufficiently 

 strong to bear the strain caused by the motion of the 

 wings in flight. In birds, however, that do not fly, such 

 as the ostrich tribe and the penguins, the dorsal vertebrae 

 are movable upon each other. 



The vertebrae between the dorsal and caudal regions of 

 the spine are amalgamated into a single bone called the 

 sacrum, which is anchylosed with the innominate bones 

 to form the pelvic arch. 



The innominate bones are always anchylosed together, 

 but are never united below, except in the ostrich. 



The caudal vertebrae, eight to ten in number, are more 

 or less movable. The last joint, which consists of several 

 vertebrae anchylosed together, is ? from its shape, termed 

 15 E. C 



