392 SMITHSONIAN MISCELLANEOUS COLLECTIONS [VOL. 50 



the adult stage ArchcBOpteryx and Hcsperorius (Marsh) had no 

 pneumatic bones at all, and Apteryx, the Odontomithes, and the 

 New Zealand moa have only a few. The development of pneu- 

 maticity in bones appears in birds to have gone hand in hand with 

 their phylogenetic development. Among recent birds the pneumatic- 

 ity of bones is in general directly correlated with the power of flight 

 and the size of the body. It is most highly developed in large birds 

 of great flying power, such as the vultures, pelicans, swans, and 

 albatrosses. Small birds, even when good fliers like the swift, usually 

 have medullated bones. As there are obviously unknown factors in- 

 fluencing the pneumaticity, besides the known factors of size and 

 flight-power, it is difficult to say beforehand to what degree the 

 bones of any bird may be pneumatic. It is noteworthy that some 

 large reptilian Sauropsida, such as the gigantic Atlantosaunts, 

 Brontosaunts, etc., had pneumatized bones with air-cavities, while the 

 bones of small reptiles are not pneumatic. This shows how important 

 the size of the body is in this respect. 



The Minute Structure oe the Air-sacs 



The thin membranes forming the walls of the air-sacs consist 

 chiefly of connective tissue composed of long, coarse, slightly wavy 

 fibrillar, to which are added, in all the sac-walls with the exception of 

 those in the pneumatic bones, spirally twisted elastic fibers (cf. 

 Leydig, 1857, p. 376). The latter render the walls of the sacs in tha 

 soft parts of the body elastic, and almost equally extensible in every 

 direction. In this connective-tissue membrane, the membrana 

 propria, the blood-vessels of the sac-wall are situated. In the walls 

 of the sacs that lie between the soft parts only few blood-vessels are 

 found (Strasser, 1877, p. 205; Baer, 1896, p. 451). In the walls of 

 the sacs situated in the bones there are, on the contrary, numerous 

 blood-vessels and a rich capillary network. The respiratory inter- 

 change of gases is therefore much more marked in the walls of the 

 sacs in bones than in those situated between the soft parts. All 

 blood-vessels of the air-sac walls belong to the general circulatory 

 system of the body, none to the pulmonary system. Of nerves only 

 a few are observed in the air-sac walls. 



Internally the sac-wall is lined with a single layer of epithelial 

 cells. In the vicinity of the ostia this epithelium is high and ciliated ; 

 farther on it gradually changes to a low pavement epithelium with- 

 out cilia. This arrangement of the epithelia in the air-sacs is well 

 shown by the appearances observed in older city pigeons, where all 



