THE RESPIRATORY SYSTEM OF MAMMALS 



153 



the branching increases. (Fig. 95.) Histological changes also 

 appear. The cartilage rings change to flat cartilage plates which 

 become smaller and smaller in the branchings until finally no 

 cartilage su])])()rts the smallest bronchioles. Patches of smooth 

 muscle arranged concentrically around the tube a])pear just within 

 the cartilage ])lates in the tunica pro])ria. As the branches decrease 

 in size the muscle increases proportionately until in the very 

 small or terminal bronchioles there is no cartilage but a distinct 



Trachea 



Bronchioh 



Fig. 95. — Bronchial tree of mammal. 



circular sheet of smooth muscle is present outside the tunica propria. 

 The epithelium changes gradually from a pseudostratified variety 

 until in the smallest bronchioles there is but one layer of columnar 

 ciliated epithelium. The goblet cells have disappeared also. The 

 glands in the tunica propria, which in the larger bronchi were so 

 numerous, have decreased so that there are none in the smallest 

 branches. The mucosa and tunica propria in smaller branches 

 show longitudinal folds. 



Terminal hronchioks (Fig. 90) are separated from the respiratory 

 tissue by their adventitia, a thin layer of fibroelastic connective 

 tissue. The tunica propria of connective tissue is rich in elastic 



