pneumatization - the formation of air cells or cavities in tissue, especially in the temporal bone.
http://www.plosone.org/article/info:doi/10.1371/journal.pone.0004497
In birds, pneumaticity of forelimb elements distal to the elbow is restricted to large-bodied forms such as pelicans, vultures and bustards (Table S3). In these birds an extensive subcutaneous diverticular network, originating from the clavicular air sac, is responsible for pneumatization of skeletal elements distant from the main pulmonary system [20]. The occurrence of pneumatic foramina in distal limb elements of ornithocheiroids and azhdarchoids [pterosaurs], and of a layer of spongy subdermal tissue in an exceptionally well-preserved fragment of wing membrane of an azhdarchoid pterosaur [16], [42], together suggest that a subcutaneous air sac system was present in at least some pterodactyloids. The primary role of such a system is likely to have been density reduction, as in birds [43], but it may have had other advantages. Differential inflation of subcutaneous air sacs along the wing membrane could have altered the mechanical properties (e.g., relative stiffness) of flight control surfaces in large-bodied pterodactyloids (Fig. 3d). In addition, this system may have assisted with thermoregulation [16], and could have also served as an intra- or interspecific signalling device during display behavior, similar to some living birds [44]. Thus, the presence of a subcutaneous air sac system likely played an important role in the functional and ecomorphological diversification of pterodactyloid pterosaurs.
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