THE LUNGS AND RESPIRATION. 127 



The blind pouches which the air cells surround are 

 called infundibula and are separated by delicate mem- 

 branous septa in which lie the capillaries of the pulmo- 

 nary artery, thus exposing the blood to the air on two 

 sides. The lung itself is supplied by the bronchial 

 arteries from the thoracic aorta and by branches of the 

 sympathetic and pneumogastric nerves. 



At birth the lungs are pinkish-white in color but in 

 later life they are marked with slate-colored patches, 

 due to the deposit in the lung tissue of particles of dirt 

 breathed in. They are light, spongy, and highly elas- 

 tic, and will float in water, crepitating upon pressure 

 owing to the air in the tissue. 



At birth, also, the lungs are solid, so that the first 

 air has to overcome adhesions between the collapsed 

 walls of the bronchioles and air sacs, but after they 

 are thus gradually unfolded, in that they are of exten- 

 sible material and open to the air above, atmospheric 

 pressure from within keeps them distended to the full 

 extent of the chest, which is air tight. They never col- 

 lapse afterwards unless puncture of the chest wall, as 

 in stabbing, causes collapse, in which case the lung 

 shrivels into a small ball. 



Respiration. That the organic materials used by the 

 body as food may give up their energy they must be 

 broken up, and for this oxygen is needed. The supply 

 of oxygen for the purpose is brought to the tissues by 

 the blood, which acquires it in the lungs, and the waste 

 product of combustion, carbon dioxide, is carried off 

 in the same manner. The lungs are, therefore, adapted 

 to take in large quantities of air and to keep up a rapid 

 exchange of oxygen and carbon dioxide in the blood. 

 This process of supplying oxygen to the tissues and 

 of removing carbon dioxide and other waste is ordi- 

 narily an involuntary act, though it can be regulated 

 temporarily, and is known as respiration or breathing. 



There are two periods to respiration; 1. inspira- 

 tion or the drawing in of air, and 2. expiration or the 



