OSTEOLOGY OF BIRDS 65 



evident disposition to contract at its base and project into the 

 pleural space. A circular pneumatic foramen is found behind the 

 transverse process on either side from the 13th to the 19th vertebra 

 inclusive. This pair of openings is largest in the 18th vertebra, 

 and smallest in the 13th. 



The " intervertebral foramina " become more circular and yet 

 smaller as we proceed toward the hinder part of the spinal column. 



In the 15th vertebra the neural crest interlocks at its posterior 

 superior angle with the anterior superior angle of the neural crest 

 of the 1 6th vertebra, by the arrowhead joint. This interlocking 

 continues throughout the series, until we arrive at the pelvis, where 

 no such joint is found to exist. The neural spines or crests 

 through this " dorsal region " of the column become gradually 

 lower and longer as we proceed toward the posterior extremity of 

 the body. 



From the 15th to the 19th vertebra inclusive, the articular facets on 

 the zygapophysial processes gradually change their direction to meet 

 the requirements of the " dorsal region" ; they, once more come to 

 face directly upwarjd anteriorly, while the reverse holds good be- 

 hind; we observe also that the transverse processes in this series 

 become longer and longer as we proceed in the same direction, and 

 their outer extremities armed in each case with a single, delicate meta- 

 pophysis which overlaps the process both before and behind it. In 

 the 15th vertebra, now under consideration, the hypapophysis loses 

 its tricornute character, and the short pedicle merely supports a 

 circular disk, with its inferior surface directed slightly forward. 

 This pedicle in the 16th becomes longer, and the disk becomes an 

 ellipse, placed longitudinally upon it. The hypapophysis on the 

 17th vertebra dips well into the pleural cavity as a laterally com- 

 pressed hook with slightly dilated apex. It is truly claw shaped in 

 the 1 8th vertebra, though still compressed from side to side, to be 

 entirely absent in the 19th. 



In Circus all the vertebrae are freely articulated upon each 

 other, from atlas to the one that first anchyloses with the ilia; in 

 Falco sparverius, however, from atlas to 13th inclusive, 

 are free, while 14th to 18th are thoroughly fused into one bone, the 

 outer angles of their diapophyses even being united by anchylosis. 



In this common " dorsal " piece of the Sparrow hawk the two 



leading vertebrae support hypapophyses. These have also fused 



together, leaving only a circular foramen between them. The 19th 



vertebra of this little falcon is free and articulates with the posterior 



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