244 THE RESPIRATORY APPARATUS 



subdivide. The walls become thinner and the car- 

 tilaginous rings disappear. The smallest bronchial 

 tubes contain muscle tissue in their walls. These 

 latter fibers, when stimulated by nerves from the 

 vagus, cause a narrowing of the caliber of the bronchial 

 tubes. When the bronchial tubes are so reduced 

 as to measure about 1 millimeter, they are termed 

 bronchioles. From the latter are given off small 

 branches which soon expand to form numbers of 

 lobules or alveoli. The central space of the alveoli is 

 called the infundibulum, and from the inner surface 

 of the alveolus, project small partitions which include 

 the air sacs or cells between them. The walls of the 

 alveolus are very thin and composed of fibro-elastic 

 tissue. The bronchi and bronchioles are lined with 

 ciliated epithelium and the alveoli and air cells by 

 flat epithelial cells, called respiratory epithelium. 



The bronchi and their subdivisions are accompanied 

 by branches of the pulmonary artery, and pulmonary 

 and bronchial veins. The pulmonary arteries upon 

 reaching the alveoli of the lungs form a capillary net- 

 work which is in intimate relation with the respiratory 

 epithelium of the air sacs, only the thin wall of the 

 capillary intervening. This permits of the ready inter- 

 change of the carbon dioxide through the wall of the 

 capillary and respiratory epithelium with the intrapul- 

 monary air, and the oxygen from the latter is absorbed 

 in the same way by the red cells of the blood in the pul- 

 monary capillaries, and returned to the left side of the 

 heart. The bronchial arteries supply the walls of the 

 larger bronchial tubes and tissue of the lungs anasto- 

 mosing with the capillaries of the bronchial and pulmo- 

 nary veins. The bronchial veins pass back to empty 

 into the azygos system on the right side and on the 

 left they drain into the superior intercostal vein. The 

 pulmonary veins are supposed to contain some venous 

 blood derived from the bronchial venous capillaries, 

 besides their arterial blood. 



