NO. 12 BODY TEMPERATURE OF BIRDS WETMORE 25 



ence of these same air-pads in the white peHcan and, possibly, in the 

 man-o'-war bird may be explained by considering that they were 

 developed as a protection while diving from a height and that they 

 have persisted now that these birds have altered their mode of securing 

 food. Similar air-pads in the screamers, however, cannot be ex- 

 plained by the same argument. Similarly there seems no adequate 

 explanation on the basis of use for pneumaticity in the pedal phalanges 

 of the Bucerotidae or in the pygostyle of the Picidae. 



Many physiologists have supposed that air-sacs have been developed 

 by birds to impart lightness to the body, especially to the bones. 

 Anatomists, however, have pointed out that while the main air-sacs 

 are more or less uniform in growth, in many cases the bones are highly 

 pneumatic in species not especially noted for strong or prolonged 

 flight. The hornbills, already cited as having the osseous system more 

 extensively permeated with ramifications of air cells than any other 

 group, are not known to be especially active on the wing. 



In the most recently published extensive account of avian air-sacs 

 Bruno Miiller ^ considers that air-sacs serve no special physiological 

 function but that they give bulk ta the bird body without adding 

 to its weight. This author continues with the statement that the 

 connection of the air-sacs with the lungs comes from their manner of 

 development, and that this connection serves merely to " assist in 

 renewing the air in the trachea." Reflection and study of the facts 

 of avian anatomy show, however, that this line of reasoning is unten- 

 able. Bats among mammals fly with the utmost ease and yet possess 

 no such system of air cells as permeates the body in birds. Some of 

 our most ancient birds from the standpoint of phylogeny are flightless, 

 have been in this condition for millions of years, and yet have as 

 perfect a system of air-sacs as are found in forms noted for their 

 powers of flight. Fossil remains of an ostrich have been found in 

 the Pliocene deposits in the Siwalik Hills in India, an indication of 

 the ancient ancestry of our present-day struthious birds. To those 

 who would adhere to the theory of Miiller as propounded above the 

 highly emphysematous condition prevailing in the screamers (Anhima 

 and Chauna) and others may seem of importance but the condition, 

 as has been said, may be ascribed more to an exaggerated develop- 

 ment, unchecked because it had no particular significance to the 

 organism as a whole. Otherwise we must expect a similar condition 

 in other species, as the Old World vultures and American buzzards 



* Air-sacs of the Pigeon, Smithsonian Misc. Coll. (Quart. Iss.) Vol. £0, 

 pt. 3, Jan. 16, 1908, pp. 403-404. 



