What is Air sac?

Air sacs are spaces within an organism where there is the constant presence of air. Among modern animals, birds possess the most air sacs , with their extinct dinosaurian relatives showing a great increase in the pneumatization in their bones. Theropods, like Aerosteon, have many air sacs in the body that are not just in bones, and they can be identified as the more primitive form of modern bird airways. Sauropods are well known for the number of air pockets in their bones , although one theropod, Deinocheirus, shows a rivalling number of air pockets.