APPABATTJS OF BESPIBATION. 73 



would surely end in the destruction of life on the globe if 

 not obviated, nature employs the respiration of plants. 



Vegetables absorb the carbonic acid gas spread through 

 the atmosphere, and under the influence of the sun's rays 

 they extract the carbon, and set free the oxygen. Thus 

 animals supply carbon to vegetables, while the latter return 

 oxygen to animals.* 



128. There exists a kind of relation between the viva- 

 city of the movements of any animal and the activity of its 

 respiration, which may be estimated by comparing the move- 

 ments of a frog and butterfly. 



129. The activity of the movements varies also in the 

 same animal according to the circumstances in which it is 

 placed; and it may be established as a general rule, that 

 whatever diminishes the vital energies tends to diminish the 

 quantity of oxygen absorbed and of carbonic acid given out ; 

 and vice versa. 



Thus in young animals during sleep and after fatigue, the 

 respiratory act is not so energetic as when the opposite 

 circumstances prevail. Let us now attend to the organs by 

 which this important function is carried on, and their various 

 modifications in different animals. 



Apparatus of Respiration. 



130. In animals of very simple organization there 

 exists no special respiratory organ. They absorb the air by 

 the general envelope of the body. This happens also in the 

 higher animals, and even to man himself ; but in all the 

 higher animals a special organ is provided for this act ; its 

 vascularity and softness, and spongy character, contrasting 

 strongly with the external integuments of the body. 



132 . Again, these organs are modified according as the 

 animal is aquatic, properly so-called, or terrestrial and aerial. 

 In the former, the organs are called gills or branchiae ; in the 

 latter, lungs or tracheae. 



132. Organs of Aquatic Respiration. Gills vary much 

 in their form. Sometimes they more resemble tufts or tuber- 

 cles, which have a texture softer than the rest of the skin, 

 and are better supplied with blood. In others, these organs 



* It might be supposed that the air of cities must be less pure in respect 

 of the amount of oxygen than that of the country; but experiment shows 

 that this is not the case. 



