ADAPTATIONS OF THE SKELETON 49 



§ 2. Adaptations to Flight 



(i) In adaptation to flight the thoracic vertebrae are 

 fused (with a few intelligible exceptions), and this fusion 

 helps to afford a firm fulcrum for the action of the wings in 

 striking the air. The wing works against the pectoral 

 girdle, with the coracoid bone braced on the breastbone, and 

 the shoulder-blade (scapula) bound by ligaments to the 

 ribs and to the backbone. There is a coherent springy 

 framework formed of backbone, breastbone, ribs, and 

 pectoral girdle ; and against this the wing has its leverage. 

 A mobile vertebral column in the thoracic region would 

 be quite inconsistent with vigorous flight, and it is inter- 

 esting to notice that there is a tendency to coalescence of 

 dorsal vertebrae in bats. In Running Birds there is no 

 thoracic fusion. 



(2) In adaptation to flight, the breastbone or sternum 

 has in most cases a keel or carina, which serves for the 

 insertion of the pectoral muscles. The absence of the keel 

 in Running Birds, and its degeneracy in the New Zealand 

 burrowing parrot Stringops, may be readily interpreted. 

 The strength of the keel in the flightless penguins is intel- 

 ligible enough when we remember how much they use their 

 fore-limbs in swimming. A breastbone or sternum is a 

 bilateral structure, arising in development from a coalescence 

 of the ventral ends of the ribs, and each side contributes 

 one side of the keel. This makes it easier to understand 

 how it is that in the crane and the whooping swan, the 

 windpipe descends into the keel and bends up again. 



It is interesting to notice that there is a keel on the 

 breastbone of bats and also in moles, both in relation to 

 the strong pectoral muscles for flying in the one case, for 

 burrowing in the other. In the extinct Pterodactyls there 

 was sometimes a slight keel ; but, apparently, never more 

 than slight. 



There is on the whole a correlation between the strength 

 of the keel in proportion to the rest of the breastbone and 



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