24 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 72 



To continue, as the statement given in the preceding paragraphs 

 becomes more clear, it seems evident that the bird owes its high 

 development, when compared with the reptile, to the growth of these 

 air-sacs as well as to a complete double circulation of the blood. The 

 truth of this statement is apparent when it is considered that the 

 Crocodilia among reptiles possess a double circulation so nearly per- 

 fect that only a comparatively small amount of venous blood finds its 

 way into the purified stream of the trunk arteries. Yet these crea- 

 tures are " cold-blooded " and become dormant when subjected to 

 cold. In other words, their body processes function so slowly that 

 when they encounter an outside temperature below a certain point 

 heat is given off by the body more rapidly than it can be produced. 

 It follows then that the bodily activities ebb lower and lower until 

 finally they are practically at a standstill. 



With animals as active as are birds means of relief from overheating 

 must be well organized ; the extension of the air-sacs through the body 

 cavity is excellent for this purpose. The walls of the sacs are very 

 poorly supplied with blood so that heat is not radiated directly by 

 means of special circulatory vessels. The thin walls of the sacs, how- 

 ever, are brought into intimate contact with the trunks bearing the 

 blood stream and their principal branches while in addition the sacs 

 closely invest the glands and organs that generate heat. It is claimed 

 that the liver produces more heat than any other organ so that the 

 blood from the hepatic drainage is warmer than any other in the body. 

 The liver itself is partly enclosed by air-sacs, while the venous 

 trunks coming from it adjoin sacs that give excellent opportunity for 

 the casting off of excess heat. Ramifications of air-sacs in the bones of 

 the body are not uniform in distribution and appear to follow no set 

 plan. Some species have the osseous system highly pneumatic 

 throughout while in others this pneumaticity is greatly reduced. 

 When air-sacs are present in bones invested by considerable muscle 

 masses they may be considered as developments that tend to further 

 the proper radiation of excess heat. Thus air cells in the keel of the 

 sternum and the coracoids would aid in controlling heat generated in 

 the pectoral muscles and supplement the work of those divisions 

 of the sacs that underlie the body of the sternum and penetrate from 

 the thorax into the cervical region. 



Evolution of the air-sacs beyond their normal development of five 

 main pairs that fill the body cavity and the cervical region apparently 

 has been partly beyond control. The presence of numerous cells 

 between the skin and muscles in brown pelicans may be supposed to 

 break the impact of the water as the birds dive for food. The pres- 



