ISO 



ELEMENTS OF PHYSIOLOGY 



upper end of the sternum ; and there it divides into two 

 bronchi, called the right bronchus and the left bronchus 

 (plural, bronchi} (Fig. 129). The bronchi subdivide into a 

 greatnumberof smaller 

 branches, called bron- 

 chial tubes. Cartilage 

 is found in the walls of 

 all but the smallest of 

 the tubes. The subdi- 

 vision continues until 

 the whole lung is pene- 

 trated by branches, all 

 having the general 

 name of bronchial 

 tubes (Fig. 128). The 

 smallest are only about 

 y 1 ^- of an inch in di- 

 ameter. They ramify 

 through the lungs, 

 somewhat like the 

 branching of a tree, 



FlG. 129. The 

 chea (front) . 



h, hyoid bone; /, t f , thy- " "*& * a> arytenoid cartilages; h, 



cartilage; c, cri- f Unnel-Shaped Chamber hyoid bone; *, t', thy- 



Tra - each tiny tube finally FlG - 130. -The Tra- 



.. . ., v chea (back). 



ending in a wider 



coid; e, epiglottis; tr, 777 /T 



trachea; b and b', Called 3. lOOUle (Fig. 

 bronchi. 



roid cartilage; c, cricoid, 

 <?, epiglottis; tr, trachea; 

 b and b 1 , bronchi. 



131), into which so 

 many dilated sacs, called air cells, open that the walls of 

 the terminal chamber, or lobule, may be said to consist of 

 tiny cups, or air cells, placed side by side. (The word 

 " cell " is here used in its original sense to denote a 

 cavity or chamber, and not in the sense of a protoplasmic 

 cell.) 



251. The wall of an air cell consists of elastic connective 

 tissue lined with a layer of very flat and thin epithelial cells 

 (Fig. 132). This lining is continuous with the epithelial 



