CHAPTER EIGHTH. 



OF RESPIRATION. 



245. FOR the maintenance of its vital properties, the 

 blood must be submitted to the influence of the air. This 

 is true of all animals, whether they live in the atmosphere 

 or in the water. No animal can survive for any considera- 

 ble period of time without air ; and the higher animals 

 almost instantly die when deprived of it. It is the office of 

 RESPIRATION to bring the blood into communication with 

 the air. 



246. Among animals which breathe in the open 

 air, some have a series of tubes branch- 

 ing through the interior of the body, 



called trachece (Fig. 89, ), and opening 

 externally upon the sides of the body, 

 by small apertures, called stigmata (s) ; as 

 in the insects and in some spiders. But 

 the most common mode of respiration is 

 by means of the LUNGS, a pair of peculiar 

 spongy or cellular organs, in the form 

 of large pouches, which are the more 

 complicated in proportion to the quantity 

 of air to be consumed. 



247. In the lower vertebrata provided with lungs there is 

 a single organ ; but in the higher classes they are in pairs, 

 placed in the cavity formed by the ribs, one on each side of 



Fig. 89. 



