234: SCIENCE OF COMMON THINGS. 



Combustion a form of respiration. Construction of the lungs. 



1562 What is the object of drawing air into the lungs and again expel- 

 ling it ? 



To oxidize the carbon and hydrogen of the blood. 



1563 We receive into the lungs oxygen through the medium of the atmo- 

 sphere, mingled with nitrogen : what do we expel from the lungs f 



The nitrogen returns unaltered ; the oxygen unites 

 with the carbon of the blood to form a gas carbonic 

 acid, and with hydrogen to form the vapor of water. 



1564 Are not these the same products of every ordinary form of com- 

 bustion f 



They are ; therefore respiration or breathing is but 

 a form of combustion. 



1565 It is estimated that a man receives into his system about eight 

 hundred pounds of oxygen from the atmosphere in a year, but his weight at 

 the end of the year has increased but little, or not at all : what becomes of all 

 this oxygen ? 



No part of it remains in the 'body, but is given out 

 again, combined with carbon and hydrogen. 



1566 How much carbon passes out of the system of an adult man by 

 the agency of respiration daily ? 



About fifteen ounces. 



1567 How is this great abstraction of material from the body made up T 



By the food which we eat. 



1568 What are the lungs ? 



Lungs are made up of bloodvessels imbedded in a 

 fleshy substance which we denominate cellular tissue, 

 and expanded over the walls of a series of chambers or 

 cavities. 



They are so situated in the thorax (or chest) that the air must enter 

 into them whenever the cavities of the thorax are enlarged. The process 

 of breathing is performed thus : When we INHALE, the thorax (or chest) 

 is expanded ; in consequence of which a vacuum is formed round the 

 lungs, and the heavy external air instantly enters (through the mouth and 

 throat) to supply this vacuum. 



When we EXHALE, the thorax contracts again; in consequence of 

 which it can no longer contain the same quantity of air as it did before, 

 and some of it is necessarily expelled. When this expulsion of air takes 

 place, the lungs and muscular fibres of the windpipe and gullet contract in 

 order to assist the process. 



1569 To what may the mechanism of the lungs in respiration be con* 

 pared ? 



To the action of a bellows. 



