OF ORGANIZED BEINGS. 409 



beneficial results of respiration. It is supposed that by 

 the action of the air on the blood, the latter in some way 

 receives an increased quantity of heat, or caloric, by which 

 means the body maintains its fixed standard of tempera- 

 ture. 



All animals furnished with lungs have the power of 

 expressing their wants, their desires, their pleasures and 

 their pains, by words or sounds peculiar to each species. 



Birds, although they are furnished with lungs as well 

 as quadrupeds, by a wise provision of nature, are enabled 

 to fill almost every part of their bodies with air. They 

 have air cells which communicate with their lungs situa- 

 ted in almost all parts of their bodies. They are situated 

 not only in the soft parts but their bones are furnished 

 with them. Mr Hunter tied up the natural air tube (the 

 Trackia) of a hen, and made an opening into one of the 

 bones furnished with a cell and the function of respiration 

 was carried on by means of it. Mr Hunter often having 

 made this and various after experiments to prove that 

 air cells did exist, thought that this was a provision which 

 nature had made, that birds being made comparatively 

 light by being tilled with air, and this after it had been 

 dilated by the natural heat of the body, would perform 

 with much more ease their aerial journeys. He after- 

 wards however found that the ostrich which does not fly, 

 was abundantly furnished with them, while the bat which 

 does, had no such peculiarity of structure. 



Insects breathe through small pores situated on the 

 sides of their bodies or on their backs. Amphibious 

 animals, or such as can live in two elements, have 

 lungs which extend the whole length of their bodies. 



Animals that reside in the water, require air as well 

 as those that live on the land. The cetaceous tribe, such as 

 the whale, are furnished with lungs like quadrupeds, but 

 most fish are provided with gills, which answer the pur- 

 pose of lungs. The water is made to pass over a small 

 membrane, and the air which it contains or the oxygen 

 of the water comes nearly in contact with the blood, and 

 thus the important change is effected. The surface that 

 is exposed to the action of the air in fish, and the blood 

 necessary to supply the wants of the body are much less 



