304 THE RESPIRATORY SYSTEM 



loose fibrous tissue which invests the outer surface of the cartilaginous 

 coat contains many small sympathetic nerve trunks and ganglia. 



THE LUNG 



At the root of the lung the trachea divides into a primary bronchus 

 for each lung. By repeated subdivisions the earliest branches being 

 given off at acute angles, the later ones at more obtuse angles the 

 smaller and smaller bronchi finally end in minute terminal bronchioles 

 which lead through the alveolar ducts to the pulmonary air sacs and 

 alveoli. One may thus distinguish between primary, secondary, and 

 tertiary bronchi, and bronchioles. The mode of division for the main 

 series of bronchi is monopodial, of the smaller bronchi a mixed dichot- 

 omy and monopody (Miller, Jour. Morph., 24, 4, 1913). According to 

 Piersol the bronchioles are the branches of the fourth or fifth order. 



BRONCHI 



The wall of the primary bronchi is similar in structure to that of 

 the trachea, but in bronchial tubes which are one or two divisions 

 removed (secondary and tertiary bronchi) from the primary bronchi 

 the plates of cartilage are no longer C-shaped, and a complete muscularis 

 mucosas, within the cartilages, forms the outermost boundary of the 

 mucous membrane. In such tubes typical bronchi the wall, as in the 

 trachea, comprises: 



1. A mucosa. 



2. A submucosa. 



3. A fibrocartilaginous coat. 



The mucosa, a continuation of that of the trachea, is lined by 

 tall, columnar, pseudo-stratified, ciliated epithelium which rests upon a 

 distinct elastic basement membrane. The epithelium is thrown into 

 wavy longitudinal folds. The tunica propria is extremely vascular; it 

 possesses an abundant supply of thin-walled veins of small caliber, to- 

 gether with many lymph vessels. Its connective tissue forms a delicate 

 fibrous reticulum in the meshes of which are many lymphocytes. The 

 outer portion of the tunica propria contains bundles of fine longitudinal 

 elastic fibers, which form a complete layer about the tube. This elastic 

 layer is thickest opposite the ridges and thinnest opposite the troughs of 

 the epithelial waves. 



The outer boundary of the mucous membrane contains a well- 



