ii SKELETONS OF BIRD AN 1 1) REPTILE 15 



The Coracoids have become much stronger, while the 

 Precoracoids and the Interclaviclc have disappeared. 

 The Scapula is formed of sound bone from end to end. 

 In shape it has grown long and blade-like — in the 

 chapter on Flight I shall explain what advantage has 

 thus been gained — and it is fastened to the back-bone 

 by muscles firmly, but in a way that allows a great 

 deal of free play. And while maintaining their posi- 

 tions relatively to one another the various bones have 

 much changed their attitudes. If we look upon the 

 shoulder-joint as the fixed point, the sternum begins 

 farther back, and, consequently, the Coracoids slope 

 forwards instead of backwards, in order, with their 

 other ends, to reach the joint, towards the formation 

 of which they contribute so much. And the bird is 

 much deeper-chested than the lizard : therefore they 

 must slope not only forwards but upwards. With all 

 these changes they retain their slope outward. Sca- 

 pula and Coracoid form an acute angle, opening 

 towards the tail, whereas, in the lizard, the angle 

 formed opens towards the head, or, sometimes, one 

 bone continues the line of the other. The clavicles, 

 moreover, have not the slope backward towards the 

 shoulder-joint that is so marked in many lizards ; 

 they point upward and outward. The most im- 

 portant result of all these changes is that a firm 

 pivot has been found on which the wing can turn — 

 a firm pivot (1) because of the great strength of 

 # the Coracoid ; (2) because both it and the Clavicle 

 have a marked outward slope ; and (3) because 

 they buttress each other. 



