Assertion: foot's bone | /r/HasProperty | fused with each other
Facets:
value: to preserve skeletal strength
type: PURPOSE
Source sentences:
text: many of bones are hollow and thin; many bones are penetrated by extensions of the air sac system and thus contain buoyant in warm air; some bones present in reptile skeletons have been eliminated; internal struts developed where stress is greatest; some bones are fused with each other to preserve skeletal strength (skeleton has rigitidy); female birds have 1 ovary; reduction of some organs; no teeth