I love my science classes, giving me inspiration to work on speculative dragon anatomy some more. XD

This time, I've explored a little into the pulmonary air sac system and how it might be helpful for draconic wing anatomy and respiration. Here is some more detail in terms of 'explaining' the decision to use the PAS system in this way for a dragon wing plan (this can be read in the comments, but this is to save viewers' time):

I have mostly been using pterosaur aerodynamics and flight physiology to help find what might work best for a dragon. In the case of the dragons I think of, they are usually about 10-15 feet in length with 30-foot wingspans (15ft for each wing). In a particular scientific article - "Respiratory Evolution Facilitated the Origin of Pterosaur Flight and Aerial Gigantism" by Leon P. A. M. Claessens, Patrick M. O'Connor, David M. Unwin (seen here http://www.plosone.org/article/info.....l.pone.0004497 ), numerous specimens have been studied, and as the animals became bigger, there has been observation of the presence of pneumatic structures in the body. The article states,

"There is a strong correlation between pneumaticity and size. Pneumaticity is generally absent in small pterosaurs, or confined to the vertebral column, but is almost always present in individuals with wingspans in excess of 2.5 metres and seemingly universal in all taxa with wingspans of 5 metres or more (Fig. 4). This suggests that density reduction via the replacement of bone and bone marrow by air-filled pneumatic diverticula likely played a critical role in circumventing limits imposed by allometric increases in body mass, enabling the evolution of large and even giant size in several clades." (Claessens, et. al.)

Subcutaneous air sacs are also seen in larger birds. To further quote the article:

"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, 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." (Claessens, et. al.)

I hope this may have had helped me explain my perspective and my decision to include this theory into my speculative dragon anatomy.

Originally pencil sketches redrawn and colored in Photoshop CS5.

Art © Stephanie Dziezyk/LeccathuFurvicael 2012. Do not redistribute, copy, alter, etc., any portion of this image without permission from me, the artist.