THE PULMONARY ALVEOLI. 



101 



The wall of a pulmonary alveolus or air sac is composed of connec- 

 tive tissue, supporting the capillary network, with a considerable 

 amount of elastic tissue and an occasional muscular fibre. The whole, 

 as we have said, is lined with a single layer of flat pavement epithe- 

 'lium. The capillary plexus, when filled with blood, affords the most 

 prominent feature of the wall; but when the vessels have been emp- 



FIG. 72. DIAGRAM OF AN ULTIMATE PULMONARY LOBULE. 



A. A terminal bronchus. 



B. The air-sacs or alveoli. 



tied of their contents, they become very insignificant under the micro- 

 scope, and the fibro-elastic tissue becomes more apparent. You will 

 have observed that, aside from the vascular supply, the histology of 

 an alveolar wall resembles very closely that of a terminal bronchus, 

 and when the vessels are all empty it is frequently difficult to differ- 

 entiate them in the mounted section. 



FIG. 73. DIAGRAM SHOWING AN ULTIMATE PULMONARY LOBULE IN LONGITUDINAL SECTION, 

 SHOWING THE MANNER IN WHICH THE ALVEOLI ARE ASSOCIATED IN CONNECTION WITH A 

 TERMINAL BRONCHUS. 



A. Terminal bronchus, entering 



B. The infundibulum. 



C. C, C. Alveoli. 



Fig. 74 shows a single alveolus, the vessels of which have been 

 injected with a solution of colored gelatin. The alveolus has been 

 divided through the middle, and shows as a cup-shaped cavity. The 

 fibrous marginal walls are indicated with their tortuous capillaries. 



