AIR VESICLES 137 



vesicles. Elastic fibrous tissue is also present everywhere in 

 the bronchial walls and is continued over the vesicles them- 

 selves. 



Bronchial tubes above Vm in. in diameter have in their 

 walls cartilaginous plates, muscular tissue, fibrous elastic 

 and inelastic tissue and a lining membrane of ciliated epi- 

 thelium. 



Bronchial tubes l /5o in. in diameter, and smaller, have in 

 their walls the same elements except the cartilage; but as the 

 tubes subdivide, their walls grow continuously thinner and 

 the inelastic tissue becomes less and less in amount until it 

 finally practically disappears; the ciliated epithelial cells 

 gradually give place to a single layer of squamous cells in 

 the smallest tubes. The smallest bronchial tubes, the bron- 

 chioles, are from ^20 to M"o in. in diameter. O'f course ev- 

 erywhere in the walls there are vessels and nerves. 



The Air Vesicles. Each bronchiole opens into a collection 

 of air vesicles, or cells, called a pulmonary lobule. The term 

 lobulette will be here applied to it, however, reserving the 

 word lobule for a collection of lobulettes about *% in. in di- 

 ameter. The bronchiole entering the lobulette becomes the 

 infundibulum (Fig. 50), a slightly dilated canal from which 

 are given off from eight to sixteen oblong vesicles, the true 

 air cells. The cells are a little deeper than they are wide and 

 end in blind extremities. The diameter of the lobulette is 

 about %o-%2 in. ; that of the vesicle about ^oo-^o in. It has 

 been estimated that there are some 725,000,000 of these ves- 

 icles in the lungs and that their combined area is something 

 over two hundred square yards. 



The walls of the air cells are very thin, being composed of 

 a single layer of -flattened epithelium together with highly 

 elastic fibrous tissue. Ramifying in this latter is a most 

 abundant supply of capillaries, which are larger here than 

 anywhere else in the body. The physical conditions are most 

 favorable for the exchange of gases between the blood and 

 air, each capillary being exposed to vesicles on both sides, 



