130 



STUDENTS HISTOLOGY 



visible, especially on the surface of the lung. By a repetition of 

 such elements the lung is constructed. 



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

 connective tissue, supporting the capillary network, with a con- 

 siderable amount of elastic tissue. The whole, as we have said, is 

 lined with a single layer of flat, pavement epithelium. The capil- 

 lary plexus, when filled with blood, affords the most prominent 

 feature of the wall ; but when the vessels have been emptied of 

 their contents, tHey become very insignificant under the micro- 



FIG. 88. TRANSVERSE SECTION OF A SINGLE PULMONARY ALVEOLUS. CAPILLARIES 

 INJECTED. STAINED WITH H^MATOLYLIN AND EOSIN (X400). 



A, A, A. Walls of the alveolus. 



B, B. Injected capillaries. 



C, C. Pavement cells lining the alveolus. These cells cover the capillaries, but do not so 

 appear in the drawing, as the latter are filled with an opaque injection. The observer is sup- 

 posed to he above the sectioned alveolus, viewing the cup-shaped cavity. 



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

 will have observed that, aside from the vascular supply, the his- 

 tology of an alveolar wall resembles very closely that of a terminal 

 bronchiole, and when the vessels are all empty it is frequently diffi- 

 cult to differentiate them in the mounted section. 



