II SKELETONS OF BIRD AND REPTILE 15 
The Coracoids have become much stronger, while the 
Precoracoids and the Interclavicle 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. 
