90 RESPIRATION. 



are pleased or caressed, they discover, by their countenance, 

 by their voice, by their movements, unequivocal symptoms of 

 cheerfulness and alacrity of mind. Thus the expressions of 

 pleasure and pain by brute animals, though not uttered in the 

 precise manner with those of the human species, are perfectly 

 analogous, and answer the same intentions of nature. 



By respiration and the instruments employed in the per- 

 formance of it, the young of animals are enabled to extract 

 milk from the breasts of the mother. By^respiration, odors 

 are conveyed to the nose ; and coughing, sneezing, yawning, 

 sighing, singing, and many other functions of the animal 

 economy, are at least partly accomplished. 



' The Respiration of Birds is carried on by an arrangement 

 of the lungs very different from that of the Mammalia. They 

 are enabled to transmit air to almost every part of their 

 bodies, by means of membranous sacks or bags, which receive 

 it from the lungs through certain orifices or passages on the 

 surface of these organs. The lungs themselves are firmly 

 attached to the ribs, and are almost incapable of dilatation or 

 contraction, but the air passes through them into the sacks 

 by the action of the abdominal muscles. In this way it is 

 diffused not only throughout the thorax and abdomen, but ex- 

 tends even to the cavities of many of the bones, which are 

 distinguished from the others by their lightness, their white 

 color, and the absence of any bloody matter or marrow in 

 their cavities. 



' This provision answers probably several important pur- 

 poses. It renders birds lighter, in proportion to their bulk, 

 than animals whose bones are filled with marrow or other 

 solid substances, and thus gives them some advantage in 

 flight ; and generally in birds of the longest and highest 

 flight, as eagles, this extension or diffusion of air is carried 

 farther than in others. But a more important object of it 

 probably is, to contribute to the muscular strength of these 

 animals, by producing a very extensive operation upon the 

 blood. The motions of birds in flight require a much greater 

 expenditure of power, than those of walking or running in 

 other animals. This power depends upon the circulation of 

 the red, arterial blood in the muscles which exert it, and in 

 order to increase the proportion of this in the system, the 

 influence of the air is carried over the whole system, instead 

 of being confined to the lungs alone. It has been found that 

 birds consume, in proportion to their size, more air than 

 quadrupeds; and this arises from its extensive influence upon 



