SKELETON. 



635 



girder principle. The texture of the bone is often very 

 spongy, and air-sacs from the lungs may be continued into 

 many of the bones, which are then more or less completely 

 destitute of marrow in adult life. In the pigeon, most of 

 the bones, except those of the tail, forearm, hand, and 

 hind-limb, contain air-spaces. Another general character is 

 the marked tendency to fusion of bones, as seen in the skull, 

 dorsal vertebrae, sacral vertebrae, ploughshare bone, carpo- 

 metacarpus, and tarso-metatarsus. 



The vertebral column is divided into five regions cer- 



S.Oc 



} 0c. 



FIG. 313. Disarticulation of bird's skull. After Gadow. 

 Membrane bones shaded. 



fi.Oc., basioccipital ; E.Oc,, exoccipital ; S.Oc., supraoccipital ; 

 Pa., parietal ; Fr., frontal ; Na., nasal ; pin., premaxilla ; .I/., 

 maxilla ; ///., jugal ; Qj., quadrato-jugal ; Qtt., quadrate ; pe., 

 periotic ; Sg., squamosal ; AS., alisphenoid ; fi.S., hasi- 

 sphenoid ; O.S., orbito-sphenoid ; Pr.SpJi., presphenoid ; TO., 

 vomer; iOS., interorbital septum; E., ethmoid; Se., nasal 

 septum; DC., dentary ; Sp., splenial ; An., angular; Sa., sur- 

 angular ; Ar., articular ; .'I/A*., Meckel's cartilage. 



vical, thoracic, lumbar, sacral, and caudal. The mobile 

 neck consists of fourteen cervical vertebrae ; from the third 

 to the twelfth these bear short ribs fused to the centra and 

 transverse processes ; the thirteenth and fourteenth have 

 them free and well developed, but not reaching the sternum. 

 Of the thoracic vertebrae, namely, those whose ribs reach 

 the sternum, the anterior three are fused to one another, 

 while the fourth is free. The complex sacral region consists 

 of the fifth thoracic (with free ribs reaching the sternum), 

 five or six lumbars, two sacrals, and five caudals, all fused. 

 Lastly, there are six free caudals ending in a pygostyle or 



