306 THE EESPIKATORY SYSTEM 



is here and there pierced by the ducts of mucous glands whose secreting 

 portions lie in the submucosa. 



The submucosa, by its broad-meshed areolar tissue, loosely unites 

 the mucous membrane to the cartilage plates. This coat contains the 

 larger blood-vessels, nerves, and lymphatics which are distributed to the 

 mucosa. It also contains the secreting portions of many tubulo-acinar 

 mucous glands, which occur in groups that in the larger bronchi almost 

 completely surround the tube. The number and size of these glands is 

 in direct proportion to the size of the bronchus. The efferent ducts of 

 the mucous glands penetrate the muscularis mucosas and open upon the 

 free surface in the interval between adjacent folds of the epithelial lining. 

 In the tunica propria the ducts possess ampullary dilatations which are 

 lined by ciliated cells and contain portions of the mucous secretion. 



The fibrocartilaginous coat is formed by a dense fibrous membrane 

 in which the cartilages are embedded. The plates of hyaline cartilage 

 vary much in number and size, being more or less highly developed in 

 proportion to the size of the bronchial tube. They possess at all times 

 a somewhat crescentic shape. In the larger bronchi three or four car- 

 tilage plates with overlapping edges encircle the entire tube. In the 

 lower mammals, e.g., the pig, these overlapping cartilages are so highly 

 developed that the plates often lie three or four deep; in man they are 

 rarely more than one or two deep. As the bronchi diminish in size by 

 division, the cartilage plates are no longer of sufficient size to completely 

 encircle the wall but leave broad intervals in which this coat is only 

 represented by fibrous tissue. In tubes of a diameter of 0.8 to 1 milli- 

 meter, bronchioles, the cartilages entirely disappear, and in these or some- 

 what smaller bronchioles the mucous glands are, likewise, no longer 

 found. According to Cutore (Anat. Anz., 47, 13, 1914), the cartilage 

 plates of the intrapulmonary bronchi contain elastic fibers, and are in 

 fact true elastic cartilages. 



The outer surface of the cartilages is invested with a clothing of 

 loose fibrous tissue of varying thickness sometimes known as the outer 

 fibrous coat in which the branches of the pulmonary artery and veins 

 and also many nerve trunks and ganglia are found. In the larger bronchi 

 the two vessels, pulmonary artery and pulmonary vein, are found on 

 opposite sides of the tube; in the bronchioles only one vessel, the artery, 

 is in relation with the tube, the vein pursuing an independent course 

 within the pulmonary tissue. 



Near the root of the lung many small lymph nodes are found in the 

 outer fibrous coat. In the smaller bronchi these are represented by 



