CHAPTER XVII* 



HOW ANIMALS BREATHE 



Arterial Blood, in passing through the system, both 

 loses and gains certain substances. It loses constructive 

 material and oxygen to the tissues. These losses are 

 made good from the digestive tract and breathing organ. 

 It gains also certain waste materials from the tissues, 

 which must be got rid of. Of these waste products, one, 

 carbon dioxide, is gaseous, and is passed off from the 

 same organ as that where the oxygen is taken in. This 

 exchange of gases between the animal and its surround- 

 ings is called respiration. 



The First Object of Respiration is to convert venous 

 into arterial blood. It is done by bringing it to the sur- 

 face, so that carbon dioxide may be exhaled and oxygen 

 absorbed. The apparatus for this purpose is analogous 

 to the one used for circulation. In the lowest animals, 

 the two are combined. But in the highest, each is 

 essentially a pump, distributing a fluid (in one case air, 

 in the other blood) through a series of tubes to a system 

 of cells or capillaries. They are also closely related to 

 each other : the more perfect the circulation, the more 

 careful the provision made for respiration, 



Respiration is performed either in air or in water. So 

 that all animals may be classed as air breathers or water 

 breathers. The latter are, of course, aquatic, and seek 

 the air which is, dissolved in the water. Land snails, 

 myriapods, spiders, insects, reptiles, birds, and mam- 



* See Appendix. 



