GENERAL CHAR A CTER1STICS. 
29 1 
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. 
