2O2 



STRUCTURE OF THE AIR-PASSAGES AND THE LUNGS. 



the amount of cartilage, and by the presence of a complete layer of circular muscle- 

 fibers; mucous glands are wanting, and the epithelium is less developed. Goblet- 

 cells secreting mucus are found as far as the smaller air-passages. 



After the small bronchi have by repeated branching become diminished in 

 diameter to from 0.5 to 0.4 mm., they are succeeded by the smallest bronchi, 

 which already bear a few alveoli on their walls. The smallest bronchi still possess 

 ciliated epithelium and unstriated muscle-fibers. 



The respiratory bronchioles are the direct continuation of the smallest bronchi. 

 In the bronchioles the cylindrical epithelium is gradually replaced, at first on one 

 side only, by small, squamous cells, and later by a mixed epithelium of large 

 plates and small, squamous cells. At the same time the mural alveoli become 

 more numerous. 



FIG. 75. Cross-section of Several Pulmonary Alveoli: A, alveolus with the blood-capillaries (c) that arise from 

 larger vessels (g g) bounding the alveoli. B, the epithelium of an alveolus: i, nucleated cells; 2, non-nu- 

 cleated platelets; 3, large, fused, non-nucleated plates. C, section of an alveolus with its epithelium and 

 subjacent capillaries. D, alveolus, with its border covered by pulmonary epithelium and plates. E, alveolus 

 whose boundary is indicated only by elastic fibers (f f). 



From these respiratory bronchioles there arise, finally, the blind, alveolar 

 ducts, which are completely lined with mixed epithelium, containing the small, 

 squamous cells only in small nests. The alveolar ducts subdivide further, and still 

 contain a few isolated muscle-fibers in their walls. These subdivisions are entirely 

 surrounded by numerous closely packed, hemispherical or spheroidal air-sacs 

 (alveoli) . 



Concerning the structure of the alveoli, the following is to be noted (Fig. 75): 

 (i) The supporting membrane of the sac is structureless, elastic, with enclosed 

 nuclei. Fine pores in the walls of the septa connect neighboring alveoli. (2) 

 Networks of numerous, fine, elastic fibers surround the air-sacs, and give to the 

 pulmonary tissue its great elasticity. As the elastic fibers are characterized by 



