THE LUNGS 231 



mencement of those which conduct it back from the 

 lungs to the left side of the heart. These cells and 

 blood-vessels are connected together by an interven- 

 ing substance of a fibrous and cellular texture. 



The blood circulating through the capillaries of 

 the body contribute to the nourishment of the animal 

 system, and furnishing all the secretions, becomes 

 changed, and is no longer capable of supporting life : 

 it becomes of a poisonous quality, in consequence of 

 having in it a too large portion of carbon. This 

 must be expelled before the blood can again be 

 rendered subservient to the purposes of life. That 

 portion of the atmospheric air called oxygen having 

 a strong attraction for carbon, unites with it whenever 

 they come in contact. The chest enlarges by means 

 of the diaphragm and the muscles between the ribs, 

 called the intercostal muscles, and others, and the 

 lungs expand with the chest in order to fill up the 

 vacuum which would otherwise exist between them 

 and the sides of the chest. These cells enlarge, and 

 a sort of vacuum is formed in each of them, as the 

 air rushes down and fills them ; and being divided 

 from the venous and poisoned blood by these mem- 

 branes alone, it is enabled to act upon the blood and 

 abstract from it the carbon, and by this means purifies 

 it, and the arterial blood is fitted for the purposes of 

 life. This purification being performed, the chest 

 contracts, and the lungs are compressed into smaller 

 compass, and a portion of air, holding in it a quantity 

 of carbon, and rendered poisonous in its turn, is 

 squeezed out. Immediately afterwards the chest 

 expands again, and the lungs expand with it, and 

 pure atmospheric air is drawn into them, which is 

 immediately thrown out again by the compression of 

 the lungs, which, like the preceding expiration, is 



