GENERAL CHARACTERISTICS 



1463 



air taken in through the windpipe. Thus, whereas, in Mammals the lungs are 

 inclosed in complete sacs (the pleuron), and are freely suspended in the cavity 

 of the chest, in the present class they are molded to the form of the back of that 

 cavity, while some of the great air tubes pass completely through them, and thus 

 carry the air to all parts of the body. In most Birds even the bones, which are 

 hollow, are thus permeated by air, and in the dried state they show a small aper- 

 ture (pneumatic foramen) by which the air tube enters. It is in consequence of 

 this arrangement that it is impossible to kill a ' ' winged ' ' bird by compressing its 

 windpipe, the process of respiration being carried on by means of the air entering 

 the broken end of the bone. In addition to the hollow bones, Birds also have a 

 number of air sacs disposed beneath the skin. Curiously enough, there appears to 

 be no sort of relation between the power of flight of a bird and the degree of de- 

 velopment of pneumaticity, as the aeration of the body and bones is called. The 

 hornbills, for instance, which are poor and heavy fliers, have the whole of the 

 bones, including the vertebrae, so hollowed that they are reduced to little more than 

 shells, while in their not very distant cousin, the rapid flying swift, the aeration is 

 reduced to a minimum. Among swimming 

 birds a similar difference may be observed, 

 the gannet having a remarkably pneumatic 

 skeleton and large air sacs, while in the allied 

 cormorants there are no air sacs, and the bones 

 are but slightly or not at all pneumatic. 

 According to the old theory, the heated 

 air in the sacs and hollow bones made the 

 bird lighter than the medium in which it flew, 

 and thus rendered flight easy; but, as Mr. 

 Headley well observes, the sight of an eagle 

 flying off with a lamb ought to convince any- 

 one that the saving of a fraction of an ounce 

 cannot make the slightest difference to its 

 flight. Moreover, the swallow has all the 

 bones solid. That the air sacs aid to some 

 extent in general respiration, and thus help 

 in maintaining the high temperature of the 

 blood in Birds (reaching in some cases 112 

 F. ) is probable, but this cannot be their sole 

 function, and it is most likely that during 



n' t. 1 u j > -L ii i v. -j i- head; 2. neck; 3. back; 4. tail: 5. ribs; 



flight, when a bird s breathing must be rapid. 6 . breastbone . 7 . fu 3 rcula . 8 4 me taco r acoid ; 

 they are the chief agents in maintaining an 9- thumb; 10. humerus; n. ulna; 12. metacar- 



,, r ,, ATAV. P us ' J 3- phalanges; 14. pelvis; 15. temur ; 16. 



equable temperature of the system. The tibia . I? . metatarus or canon ^ . I8 . toes . 

 function of the pneumaticity of the bones is 



not at present decided, and it would therefore be only entering on controversial 

 matters to discuss it here. That one of the objects of the coat of feathers, which 

 forms a most efficient insulator, is to assist in the maintenance of a uniform high 

 temperature, cannot be doubted. 



SKELETON OF VULTURE. 



