THE MACHINERY OF FLIGHT 69 
The Tern and the Falcon, on the other hand, have 
short, deep keels, and therefore short but big and 
strong flight muscles. Accordingly, their ordinary 
stroke is short but powerful. It is highly important 
also that there should be a firm pivot on which the 
wing may rest as it moves. During part of the 
wing’s down-stroke there must be considerable 
Fie. 19. 
Breastbone and connected bones of Adjutant—actual length of 
keel 43 inches. Cl.: Clavicle; Co.: Coracoid; Se.: Scapula. 
pressure inward, and were the skeleton to give, 
were it to supply only an unsteady wobbling pivot, 
vigorous flight would be out of the question. How 
is the pivot formed ? There is a strong bone, the 
coracoid, which springs from the front part of the 
breastbone and points forward and outward. It 
