RESPIRATION 77 



by which gaseous materials are made to pass between the 

 body and the atmosphere is kjiown as 



Respiration. As usually denned, respiration, or breath- 

 ing, consists of two simple processes that of taking air 

 into special contrivances in the body, called the lungs, and 

 that of expelling air from the lungs. The first process 

 is known as inspiration; the second as expiration. We 

 must, however, distinguish between respiration by the 

 lungs, called external respiration, and respiration by the 

 cells, called internal respiration. 



The purpose of respiration is indicated by the changes 

 that take 'place in the air while it is in the lungs. Air 

 entering the lungs in ordinary breathing parts with about 

 five per cent of itself in the form of oxygen and receives 

 about four and one half per cent of carbon dioxide, con- 

 siderable water-vapor, and a small amount of other im- 

 purities. These changes suggest a twofold purpose for 

 respiration : 



1. To obtain from the atmosphere the supply of oxygen 

 needed by the body. 



2. To transfer to the atmosphere certain materials 

 (wastes) which must be removed from the body. 



The chief organs concerned in the work of respiration 

 are 



The Lungs. The lungs consist of two sac-like bodies 

 suspended in the thoracic cavity, and occupying all the 

 space not taken up by the heart. They are not simple 

 sacs, however, but are separated into numerous divisions, 

 as follows : 



1. The lung on the right side of the thorax, called the 

 right lung, is made up of three divisions, or lobes, and 

 the left lung is made up of two lobes. 



2. The lobes on either side are separated into smaller 



