STRUCTURE OF THE AIR-TUBES. 



899 



pulmonary parenchyma. The prevailing form of division is dichotomous ; 

 but sometimes three branches arise together, and often lateral branches are 

 given off at intervals from the sides of a main trunk. The larger branches 

 diverge at rather acute angles, but the more remote and smaller ramifications 

 spring less and less acutely. After a certain stage of subdivision each bron- 

 chial tube is reduced to a very small size, and, forming what has been 

 termed a lobular bronchial tube, enters a distinct pulmonary lobule, within 

 which it undergoes still farther division, and at last ends in the small 

 cellular recesses named air-cells or pulmonary cells. 



Within the lungs the air-tubes are not flattened behind like the bronchi 

 and trachea, but form completely cylindrical tubes. Hence, although they 

 contain the same elements as the larger air-passages, reduced gradually to a 

 state of greater ar.d greater tenuity, they possess certain peculiarities of 

 structure. Thus, the cartilages no longer appear as imperfect rings running 

 only upon the front and lateral surfaces of the air-tube, but are scattered 

 over all sides of the tube in the form of irregularly shaped plates of various 

 sizes. These are most developed at the points of division of the bronchia, 

 where they form a sharp, concave ridge projecting inwards into the tube. 

 They may be traced, becoming rarer and rarer and more reduced in size, as 

 far as bronchia only one-fourth of a line in diameter, beyond which the tubes 

 are entirely membranous. The Jibrous coat extends to the very smallest 

 tubes, becoming thinner by degrees and degenerating into areolar tissue. 

 The mucous membrane, which extends throughout the whole system of air 

 passages, and is continuous with that lining the air-cells, is also thinner 

 than in the trachea and bronchus, but it retains its ciliated columnar 

 epithelium. The yellow longitudinal bundles of elastic fibres are very dis- 

 tinct in both the large and small bronchia, and may be followed by dissection 

 as far as the tube can be laid open, and by the microscope into the smallest 



Fig. 627. 



Fig. 627. PORTION OP THE 

 OUTER SURFACE OF inn 

 Cow's LUNG (from Kol- 

 liker after Hurting. ~ 



a, pulmonary vesicles filled 

 artificially with wax ; 6, the 

 margins of the smallest lobules 

 or infuudibula. 



tubes. The muscular 

 fibres, which in the trachea 

 and bronchi are confined 

 to the back part of the 

 tube, surround the bron- 

 chial tubes with a con- 

 tinuous layer of annular 

 fibres, lying inside the 

 irregular cartilaginous 

 plates ; they are found, however, beyond the place where the cartilages 

 cease to exist, and appear as irregular annular fasciculi even in the smallest 

 tubes. 



Air-cells or Pulmonary vesicles. These cells, in which the finest ramifi- 

 cations of each lobular bronchial tube ultimately terminate, are in the 

 natural state always filled with air. They are readily seen on the surface 

 and in a section of a lung which has been inflated with air and dried ; 



3 N 2 



