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CHAPTER XL 



RESPIRATION. 

 1. Respiration in General. 



THE acticfti of atmospheric air is equally necessary for 

 the maintenance of animal, as of vegetable life; and as the 

 ascending sap of the one cannot be perfected unless exposed 

 to the chemical agency of air in the leaves, in like manner 

 the blood of animals requires the perpetual renovation of 

 its vital properties by the purifying influence of respiration. 

 The great importance of this function is evinced by the con- 

 stant provision which has been made by Nature, in every 

 class of animals, for bringing each portion of their nutritive 

 juices, in its' turn, into contact with air. Even the circula- 

 tion of these juices is an object of inferior importance, com- 

 pared with their aeration; for we find that insects, which 

 have but an imperfect and partial circulation of their blood, 

 still require the free introduction of air into every part of 

 their system. The necessity for air is more urgent than 

 the demand for food; many animals being capable of sub- 

 sisting for a considerable time without nourishment, but all 

 speedily perishing when deprived of air. The influence of 

 this element is requisite as well for the production and de- 

 velopment, as for the continuance of organized beings in a 

 living state. No vegetable seed will germinate, nor will 

 any egg, even of the smallest insect, give birth to a larva, 

 if kept in a perfect vacuum. Experiments on this subject 

 have been varied and multiplied without end by Spallanza- 

 ni, who found that insects under an air pump, confined in 

 rarefied air, in general lived for shorter periods in proper- 



