Ratite birds are strictly flightless and their anatomy reflects specializations for terrestrial life. The term "ratite" is from the Latin word for raft, "ratis", because they possess a flat breastbone, or sternum, shaped like a raft. This characteristic sternum differs from that in flighted birds, where the pectoral musculature is disproportionately large to provide the power for wingbeats and the sternum develops a prominent keel, or carina sterni to anchor these muscles. The clavicles do not fuse into a furcula. Instead, if present at all, each is splint-like and lies along the medial border of the coracoid, attached there by a coraco–clavicular ligament. There is an obtuse angle between the scapula and coracoid, and the two bones fuse together to form a scapulocoracoid. Ratites have reduced and simplified wing structures and strong legs. Except in some rhea wing feathers, the barb filaments that make up the vanes of the feathers do not lock tightly together, giving the plumage a shaggier look and making it unnecessary to oil their feathers. Adult ratites have no preen gland (uropygial gland) that contains preening oil.