238 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



of the other vertebrae, forms the vertebrarterial canal in which 

 certain soft structures run. 



The first and second vertebrae, atlas and axis, are very small and 

 possess neither ribs nor transverse processes. The atlas (1) is a 

 ring, without anterior zygapophyses. Over its centrum's upper 

 surface a small peg, the odontoid process, projects from the front 

 end of the centrum of the axis (2). The occipital condyle fits 

 into an articular pit formed above by the odontoid process, and 

 below by the atlas-centrum. 



The odontoid process is at first a distinct bone. It is part of the atlas- 

 centrum which fuses with that of the axis. 



The thoracic vertebrae bear free thoracic ribs, and the commence- 

 ment of the series is marked by the first vertebra bearing ribs 

 which unite ventrally with the sternum. The four anterior 

 vertebra are closly united with one another by their centra and 

 the processes of their arches. The thoracic ribs are made up of 

 two segments, one, the vertebral rib, similar in character and mode 

 of attachment to the last cervical rib, the other, sternal rib, arti- 

 culated distally, with a facet on the sternum. The " sacrum " of 

 the adult bird is made up of vertebras belonging to four regions 

 closely fused together. The single thoracic vertebra is easily le- 

 cognised by its rib. It possesses a large bi laminar hypapophysis, 

 and its stout transverse processes abut against the ilia. The 

 lumbar vertebra have short and strong, the sacral, lamellar trans- 

 verse processes, while the first caudal possesses a ventral caudal 

 rib as well as a dorsal transverse process. 



The free caudal vertebrae are small, and their transverse pro- 

 cesses flattened. 



The ploughshare bone is a laterally flattened plate, bent up 

 sharply on the rest of the vertebral column. 



(c) The sternum (Fig. 69) is a very broad, plate-like bone, 

 concave above, supporting the ventral wall of the thorax and 

 part of the abdomen. It is produced in front into an obtuse 

 process, the manubrium, and behind into a narrow plate, the 

 middle xiphoid process. At the sides are paired internal and 

 external xiphoid processes. On each side of the front of the ster- 

 num there is an excavation for the end of the coracoid, and 

 behind this comes the costal process, bearing four facets for the 

 .articulations of the sternal ribs. The ventral side of the sternum 



