CHAPTER EIGHTH. 



or RESPIRATION. 



245. For the maintenance of its vital properties, the alood 

 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 considerable period 

 of time without air ; and the higher animals almost instantly 

 die when deprived of it. It is the office of respiration to 

 br-ng the blood into communication with the air. 



246. Among animals which breathe in the open air, 

 some have a series of tubes branchmg 

 through the interior of the body, called 

 trachea;^ (Fig- ®9, ^,) opening externally 

 upon the sides of the body, by small aper- 

 tures, called stigmata, [s ; ) as in insects 

 and in some spiders. But the most com- 

 mon mode of respiration is by means of 

 Lungs, a pair of peculiar spongy or cel- 

 lular organs, in the form of large pouches, 

 which are the more compl cated in pro- 

 portion to the quantity of air to be con- 

 sumed. 



247. In the lower vertebrata, provided with lungs, they 

 form 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. 



