COLUMBA. ' 371- 



five thoracic vertebrae bear ribs which articulate distally with 

 the sternum. Each rib has a longer vertebral part and a 

 shorter external part, and the former has two heads arti- 

 culating with its vertebra. The first four have short 

 uncinate processes. The capitulmn of the rib articulates 

 with the centrum of the vertebra and the tuberculmn with 

 the transverse process. The first three thoracic vertebrae 

 are fused, the fourth is free, while the fifth is involved in 

 the sacrum. 



In the young bird there are five free lumbar vertebrcE 

 and then two sacral to which the ilium is attached, but as 

 development proceeds the ilium grows forwards and becomes 

 attached to all the lumbar and to the fifth thoracic. Simi- 

 larly there are in the young bird fifteen free caudal vertebrae, 

 and the ilium gradually grows backwards and fuses with ^y^ 

 of these. Of the other ten the last four fuse together to 

 form \\\Q pygostyle. 



This means that the young bird presents us with a 

 reptilian-like condition of the vertebral column in which 

 all the vertebrae are free. They consist of — 



Cervical 14 



Thoracic 5 



Lumbar 5 



Sacral 2 



Caudal 15 



The modifications then take place as age advances. 



I. The first three thoracic become ankylosed or fused. 

 2. The last four caudal become ankylosed to form the 

 pygostyle. 3. The ilium grows forwards and fuses with all 

 the lumbar and the last thoracic, and backwards to include 

 five caudals. 



Cervical. T horac ic. Lumbar. Sacral. Caudal. 



14 I Jj + I + I + 5 - f- 2 + 5 + 6 -^ \J\ 



Y 



These fusions are supposed to be a recapitulation of 

 similar modifications which have taken place gradually in 

 the descent of birds from reptiles and in adaptation to the 

 gradual adoption of flight and bipedal progression. It will 

 be remembered that a similar fusion of vertebrae into an 

 anterior vertebral plate is found in the skate, in which the 



