EFFECTS OF RESPIRATION 



487 



c- 



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 T ^ 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 

 lw tliP n'rrlif "hpprf a 



ngnt neart a 

 monary artery, which contain 



dark venous blood. As the blood traverses the pulmonary capillaries 

 around the air-cells it gives off 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. 



201. Sack-like Ends of a Bronchiole 



A, Bronchial Tube divided. B, Infundibulum or Terminal 

 -rml Sac> C Air - cells - D > Artei T breaking up into Capillaries 



pul- around and betw een E, Air-ceils. 



EFFECTS OF BESPIRATION 



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 towns; 

 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. 



