The Framework of the Bird 79 



lungs, and other organs. Look at the two neck-bones of 

 the ostrich in Fig. 47, where in addition to the central 

 aperture, through w^hich the spinal nerve-cord passes, two 

 other openings will be seen, one on each side. Through 

 these the vertebral arteries carry their burden of pure 

 blood. The outer wall of this bony canal, extending up 

 the whole length of the neck, is formed principally by 

 what is left of the ribs which were once long and free, 

 like their fellows farther down the back. Though re- 

 duced to a tiny fragmentary arc of bone, yet they still 

 perform a protecting function. 



In Archceopteryx (Fig. 5) there existed well-developed 

 abdominal ribs, exactl}^ like those found in crocodiles 

 and other reptiles. In no living bird, however, are these 

 found. 



Breast-bone 



The lower portions of the true ribs of our chicken 

 are separate pieces of bone, slanting in a forward direc- 

 tion and attached by a movable joint to the upper parts. 

 These end close together along the sides of the large 

 breast-bone, or sternum as it is called. In fact the origin 

 of the sternum can be traced to the fused ends of these 

 ribs, and in the sternum of an immature ostrich (Fig. 

 58) the line of juncture between the two lateral halves is 

 still distinct or even open. To the edge of this bone, 

 nearest the head, two column-like shoulder-bones are 

 attached, and in some birds the wish-bone is also joined 

 to it (Fig. 103). 



The sternum is one of the largest single bones in 



