576 THE TISSUES. 



Their appearance in twigs T -J ^ to T J T of an inch in diameter is 

 shown by Fig. 401. Finally the bronchi end in the lobular passages. 

 The mucous membrane lining the bronchi and 

 Fig. 401. their subdivisions is at first like that of the tra- 



chea, but 1 gradually becomes extremely thin in 

 tubes of less than ^ of an inch. It everywhere 

 consists of a layer of elastic fibres, a basement- 

 membrane goVir to ?oW of an inch thick, and a 

 ciliated epithelium. The last, even down to tubes 

 1 line in diameter, contains several layers of cells; 

 but is finally reduced to a single layer of conoidal 

 ciliated cells, ^Vo" of an inch long. In the larger 

 branches, racemose mucous glands are also found, 

 rangement of the mus- but these are wanting in tubes of less than 1 to 

 l line in diameter. The subdivisions of the 



a branch. From a man bronchial tubes do not anastomose. The air-cells 



aged 50. (Magnified 2 .... 1 -i i i T -i 



diameters.) will be described in speaking more particularly 



of the lobules. 



2. The pulmonary arteries enter the substance of the lung in 

 company with the bronchi, and follow their subdivisions also, 

 though more frequently dividing dichotomously, and hence more 

 rapidly diminishing in size. Finally, the terminal branches occa- 

 sionally, but not regularly, anastomosing, merge exclusively into 

 the capillary plexus of the air-cells in each lobule to be described 

 further on except a few fine branches to the pleura. (ITenle.} The 

 pulmonary veins rise from this plexus, in radicles more superficial 

 than the arteries, and more external in the smallest lobules; and 

 unite to form larger trunks, proceeding, in great part isolated from 

 the arteries, through the pulmonary substance. The bronchial arte- 

 ries are distributed ls, to the larger bronchial tubes ; 2c%, to $he 

 pulmonary veins and arteries, as their vasa vasorum (to the latter 

 even ^ of an inch in diameter) ; and, 3c?/y, to the pleura pulmonalis. 

 They do not go to the mucous membrane at all, and do not anasto- 

 mose with the pulmonary artery or vein. The lymphatics are nume- 

 rous, but require no distinct description. The bronchial lymphatic 

 glands are both numerous, and colored dark brown or black by a 

 carbonaceous deposit. The nerves, from the pneumogastric and the 

 sympathetic, are furnished in the interior of the lung with micro- 

 scopic ganglia, and may be traced nearly to the termination of the 



