312 



ORGANS OF RESPIRATION. 



0.85 mm. in diameter. .The epithelium of the bronchi of medium 

 size (up to 0.5 mm. in diameter) consists of a ciliated epithelium 

 having three strata of nuclei. Kolliker (81) distinguishes a deep 

 layer of basilar cells, a middle layer of replacing cells, and a super- 

 ficial zone consisting of ciliated and goblet cells. The number 

 of the last varies greatly. Glands are found only in bronchial 

 twigs that are not less than I mm. in diameter ; as in the trachea, 

 they are branched tubulo-alveolar glands of the mucous variety. 

 In these structures the mucosa contains a large number of elastic 

 fibers, the greater part of which have a longitudinal direction. 

 Furthermore, numerous lymph-cells are found, and here and there 

 a lymph-nodule. The muscularis presents, as a rule, circular fibers, 

 which do not, however, form a continuous layer. 



The smaller bronchi subdivide into still finer tubules of less 

 than 0.5 mm. in diameter (bronchioles), which contain neither car- 



stratified cili- 

 ated columnar 

 epithelium. 



Elastic fibers, 

 cut trans- 

 versely. 



Gland. 



Fig. 247. Transverse section through human bronchus ; X 2 7 



tilage nor glands. The stratum proprium, as well as the external 

 connective-tissue sheath, becomes very thin ; and the epithelium 

 now consists of but one layer, but is still ciliated. 



