234 THE HUMAN BODY, 



rounding atmosphere. In the lungs the pulmonary capil- 

 lary blood-vessels form a very close network: through their 

 walls the blood gives off to the air in the lungs carbon 

 dioxide, and takes from this air oxygen. The air in the 

 lungs consequently needs renewal from time to time: other- 

 wise it would no longer have oxygen to give to the blood, 

 iind would become so loaded with carbon dioxide as to 

 40 longer take that waste product from it. This renewal 

 is effected by the working of a system of muscles, bones, 

 and cartilages whose co-operation brings about that alter- 

 nating expansion and contraction of the chest which we call 

 breathing. When the chest contracts, air deprived of its 

 oxygen and polluted with wastes is expelled from the lungs; 

 and when it expands, fresh air, rich in oxygen, and con- 

 taining hardly any carbon dioxide, is taken into them. 



The respiratory organs are, therefore, (1) the lungs; (2) 

 the air-passages; (3) the vessels of the pulmonary circula- 

 tion, including the pulmonary artery bringing the blood to 

 the lungs, the pulmonary capillaries carrying it through 

 them, and the pulmonary veins conveying it from them; 

 (4) the muscles, bones, and gristles which are concerned in 

 producing the breathing movements.* 



The Air-Passages. Air reaches the pharynx through 

 the nose or mouth (Fig. 1) : on the ventral side of the pharynx- 



What vessels form a close network in the lungs? What takes 

 place through the walls of these vessels? Why must the air in the 

 lungs be renewed? How is the renewal brought about? What is 

 breathing? What happens when the chest contracts? What when 

 it expands ? 



Enumerate the respiratory organs. 



Through what passages does air reach the pharynx? 



* To these should be added (5) the nerve centres and nerves which control 

 the muscles of respiration, and which will be subsequently considered (see 

 Chap. XX). 



