GENERAL CHARACTERISTICS. 



291 



are freely suspended in the cavity of the chest, in the present class they are 

 moulded 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 aperture (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 development 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 anyone 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 respira- 

 tion, 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 flight, when a bird's breath- 

 ing must be rapid, they are the chief agents 

 in maintaining an equable temperature of the 

 system. The 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. 



An important structural difference between Mammals and Birds is to be found in 

 the absence in the latter of the partition or diaphragm, which in the former separates 

 the cavity of the chest containing the heart and lungs from that of the abdomen. 



SKELETON OF VULTURE. 



1, head ; 2, neck ; 3, back ; 4, tail ; 5, ribs ; 

 6, breast-bone ; 7, furcula ; 8, metacoracoid ; 

 9, thumb ; 10, humerus ; 11, ulna ; 12, meta- 

 carpus ; 13, phalanges ; 14, pelvis ; 15, femur ; 

 16, tibia ; 17, metatarsus or cannon-bone ; 18, 

 toes. 



