CHAPTER X. 

 RESPIRATION AND TEMPERATURE. 



97. THE object of respiration is to liberate the carbonic 

 acid accumulated in the blood returning from the tissues, and 

 to take in a fresh supply of oxygen, which, passing to the 

 tissues, is used in chemical decompositions, by which more 

 carbonic acid is produced. Respiration, therefore, is not a 

 process of combustion, but it affords an index of the amount 

 of combustion taking place in the tissues. It consists essen- 

 tially of an interchange of gases between the blood and the 

 medium in which the animal lives, and requires that these 

 two should be brought into as close contact as possible. 



This contact is achieved in some animals by introducing 

 the medium into the body, and conveying it to the tissues. 

 Thus, in star fishes there is a water-vascular system, and in 

 insects there are air tubes or trackece kept open by a spiral 

 thread coiled round them, which, opening by stigmata on the 

 sides, ramify throughout the whole body, and are emptied 

 and refilled with air by pulsatile movements of the abdomen. 

 But in the majority of animals, including all vertebrates, 

 the blood is brought into contact with the surrounding 

 medium in a special organ devoted to the purpose; and this 

 in water-breathing animals consists of gills or projecting 

 organs with blood-vessels on the surface, while in air-breath- 

 ing animals it consists of lungs or bags into which the air 

 is introduced. 



In frogs and serpents, the lungs are simple pouches with 

 shallow recesses round about, the partitions of which project 

 into the interior; and these pouches are each opened abruptly 

 into by a main air tube or bronchus. In turtles and croco- 

 diles the air tubes are branched, and each branch opens into 

 a cavity in the heart of a sponge of ramifying recesses ; 



