SECTION VIII 



SUPPLY OF NOURISHING MATERIAL TO BLOOD AND LYMPH, 

 AND ELIMINATION OF WASTE MATTER FROM THEM 



KESPIRATION 

 I. EXTERNAL RESPIRATION 



IF an animal be placed in a closed chamber filled with 

 ordinary atmospheric air which contains by volume 79 parts 

 of nitrogen and 21 parts of oxygen, and if the air is examined 

 after a time, it will be found that the oxygen has diminished 

 in amount, and that a nearly corresponding amount of carbon 

 dioxide has been added. 



The same thing occurs in aquatic animals the water 

 round them loses oxygen and gains carbon dioxide. An 

 animal takes up oxygen and gives off carbon dioxide. This 

 is the process of external respiration. 



I. Respiratory Mechanism. 



In aquatic animals the mechanism by which this process 

 is carried on is a gill or gills. Each consists of a process 

 covered by a very thin layer of integument, just below which 

 is a tuft of capillary blood vessels. The oxygen passes from 

 the water to the blood ; the carbon dioxide from the blood 

 to the water. 



A lung is simply a gill or mass of gills, turned outside in 

 with air instead of water outside the integument. While in 

 aquatic gill-bearing animals there is constantly a fresh supply 

 of water passing over the gills, in lung-bearing animals the 

 air in the lung sacs must be exchanged by some mechanical 

 contrivance. 



The lungs consist of myriads of small thin-walled sacs 



attached round the funnel-like expansions in which the air 



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