EFFECTS OF RESPIRATION 



487 



of the lungs. They are surrounded by a close net-work of minute blood- 

 vessels (capillaries), and it is here that the aeration of the blood takes 

 place. The air vesicles have a diameter of from yoo^ to ^^ inch, and 

 their number has been esti- 

 mated to be 750,000,000 in 

 man, whilst it must be at least 

 three times as many in the 

 horse. Their walls are com- 

 posed chiefly of elastic fibres 

 loosely interwoven, which con- 

 fer upon the lung tissue the 

 great elasticity it possesses, and 

 the epithelium lining them is 

 no longer columnar and ciliated, 

 as in the bronchioles, but flat 

 and thin, thereby permitting 

 the free passage of gases 

 through them. 



The distribution of blood 

 through the lungs is effected 

 by the right heart and pul- 

 monary artery, which contain 



dark venous blood. As the blood traverses the pulmonary capillaries 

 around the air-cells it gives ofi" carbon dioxide to, and takes up oxygen 

 from, the air in the alveoli, and is then returned to the left heart as 

 aerated blood by the pulmonary veins. 



Sack-like Ends of a Bronchiole 



A, Bronchial Tube divided. B, Infundibulum or Terminal 

 Sac. c, Air-cells. D, Artery breaking up into Capillaries 

 around and between E, Air-cells. 



EFFECTS OF EESPIEATION 



Composition of the Air. — The composition of the atmosphere has 

 been proved, by the analyses of many chemists, to be nearly uniform in 

 all parts of the world, excepting in regard to the quantity of watery 

 vapour it contains, which is, of course, far less in cold dry regions than 

 in those which are moist and warm. Pure, dry air is a mixture of 21 

 parts of oxygen, 79 parts of nitrogen, and a small proportion of carbon 

 dioxide, amounting to 1 part in every 2500 parts of this mixture. In 

 addition to the constituents above mentioned, smoke, containing com- 

 pounds of sulphur as indicated by the tarnishing of silver, and ammonia, 

 with accidental impurities derived from factories, are found near touiis; 

 and in the country at certain seasons of the year, and in certain localities, 

 the pollen of plants and micro-organisms also impair its purity. 



