374 OF KESPIRATION. 



thus included in different portions of the frame, also serves, like that 

 contained in the air-sacs of Insects, as a reservoir for the supply of the 

 principal aerating organs during active flight, when the respiratory- 

 movements are less free. 



673. The mechanism of Respiration in Birds is very different from 

 that which produces the respiratory movements in Mammalia. The 

 cavities of the chest and thorax are not yet separated by a diaphragm ; 

 except in a very small number of species, that approach most nearly to 

 the next class. But, on the other hand, the whole cavity of the trunk 

 is more completely enclosed in a bony casing, the ribs being connected 

 with the sternum by osseous prolongations from the latter, instead of 

 by cartilages, and the sternum itself being so largely developed, as to 

 cover almost the entire front of the body. Now the natural condition 

 of this bony framework is such, that, when no pressure is made upon it, 

 the cavity it encloses is in a state of distension ; and the state of 

 emptiness can only be produced by a forcible compression of the frame- 

 work, through an exertion of muscular power. The lungs, instead of 

 being freely suspended in the cavity of the chest, as in Mammalia, are 

 attached to the ribs ; and their own tissue is endowed with a degree of 

 elasticity, which causes them to dilate when they are permitted to do 

 so. In the state of distension, therefore, which is natural to the cavity 

 of the trunk, the lungs are expanded, and fill themselves with air, 

 which they draw in through the trachea ; and this condition they 

 retain, until, by the action of the external muscles upon the bony 

 framework, the cavity of the trunk is diminished, and the air is expelled 

 from the lungs and air-sacs, which are again filled as soon as the pres- 

 sure is taken off. As the air-sacs chiefly communicate with the part of 

 the lungs that is most distant from the trachea, the air has to traverse 

 the whole extent of those last organs, both when it is being drawn into 

 the air-sacs, and when it is being expelled from them ; so that it is 

 made to serve for the aeration of the blood in the most effectual manner. 



674. Thus, although the respiratory apparatus of Birds does not 

 possess the highly concentrated development which we shall find it to 

 present in Mammals, it serves, by the extension of the aerating surface 

 through the body, to bring the air and the blood into most intimate 

 relation ; and the energy of the function is further provided for, by the 

 mode in which the pulmonary circulation is carried on (a distinct heart, 

 as it were, being provided for it, 564), as well as by the arrangement 

 of the blood-vessels, which transmit to the respiratory organs the whole 

 of the blood that has been returned in a carbonated state by the great 

 veins of the system. The very large proportion of red corpuscles con- 

 tained in the blood gives additional effect to these provisions. The 

 very high amount of respiration which is natural to Birds, and which 

 cannot be suspended even for a short time without fatal consequences, 

 has a direct relation (as already explained) with their extraordinary 

 muscular activity, as well as with the high bodily temperature which 

 they are fitted to maintain, and which cannot be lowered in any great 

 degree without the suspension of their other functions. Birds are 

 peculiarly susceptible of impurities in the atmosphere ; and it has been 

 shown by experiment, that if a Bird, a Mammal, and a Reptile, be 



