86 PHYSIOLOGY. 



the outer air by the bronchial twigs, the bronchi, and the 

 trachea. 



2. The Pulmonary Capillaries, forming a thick network 

 around and between the air sacs. These capillaries receive 

 their blood from the pulmonary artery, and return it to the 

 heart by the pulmonary veins. 



Elastic Tissue in the Lungs. The air vesicles, with 

 their supplying air tubes and their surrounding blood tubes, 

 are bound together by elastic tissue, which fills up most of 

 the intervening space. 



The Windpipe or Trachea. The windpipe has in us 

 walls C-shaped cartilages, with the open part of the C 

 on the dorsal surface. These cartilages continue in the 

 bronchi, and so on until in the smaller twigs they finally 

 disappear. The cartilages are held together, and the 

 dorsal gap of the cartilages (the gap would be like that of 

 a series of horseshoes piled one on top of another) bridged, 

 by tough fibrous tissue, with much elastic tissue, and 

 with plain muscle fibers ; the plain muscle fibers are very 

 abundant in the smaller air tubes. 



The Mucous Membrane. The lining of the trachea 



is a mucous mem- 

 brane. It pours 

 out on its surface 

 a substance some- 

 what like white of 

 egg, called mucus. 

 This keeps the air 

 300) ' moist, and catches 



particles of dust that are in the inspired air. There is a 

 constant slow current of mucus toward the throat, whence 

 it is, from time to time, hawked up. 



