24 CIRCULATION. RESPIRATION. [iNTROD. 



polygastrica, there is no good evidence of the existence of any 

 circulation at all ; their textures imbibing the fluid in which they 

 live. 



The presence of atmospheric air is necessary to the existence of 

 all organized beings. The air both passes by endosmose into their 

 nutrient fluids, and receives from them certain deleterious gases 

 developed in their interior. The function, by which the fluids are 

 thus aerated, is called Respiration. In plants, the introduction of 

 atmospheric air conveys nutriment to the organism ; carbonic acid 

 and ammonia are thus introduced ; the former is decomposed, its 

 carbon is assimilated, and its oxygen is exchanged for a fresh sup- 

 ply of atmospheric air. As the agent in the decomposition of the 

 carbonic acid is light, it is evident that the generation and the evo- 

 lution of oxygen can take place only in the day-time. Consequently, 

 during the night, the carbonic acid, with which the fluids of the 

 plant abound, ceases to be decomposed, and is exhaled by its leaves. 

 Hence, plants exhale oxygen in the day-time, and carbonic acid at 

 night. 



In animals, carbonic acid accumulates in the blood during its 

 circulation ; and, when the atmosphere is brought to bear upon the 

 capillary vessels containing the blood charged with this gas, a 

 mixture takes place through the delicate walls of the vessels, the 

 atmospheric air passing in, and carbonic acid, with nitrogen and 

 oxygen, in certain proportions, escaping. Thus the evolution of 

 carbonic acid, and the absorption of oxygen and nitrogen, are the 

 characteristic features of respiration in animals. 



It is highly interesting to notice, how plants are thus subservient 

 to the well-being of animals, in the respiratory function, as well 

 as in preparing nutriment for them. By their respiration they serve 

 to purify the air for animals ; for, in absorbing the carbonic acid 

 from the atmosphere, they are continually depriving it of an ele- 

 ment which, if suffered to accumulate beyond certain bounds, would 

 prove destructive to animal life. 



From the fluids of animals and plants, certain materials are se- 

 parated by a singular process, nearly allied in its mechanism to 

 nutrition, and called the function of Secretion. The secreted matters 

 are various, and have very different ends : in some cases being des- 

 tined for some ulterior purpose in the ceconomy ; in others, forming 

 an excrement, the continuance of which in the organism would be 

 prejudicial to it. 



The function, which has for its object the propagation of the 

 species, Generation, presents many points of resemblance in plants 



