152 THE RESPIRATORY SYSTEM 



just back of the larynx. As it enters the thorax, the trachea 

 divides into two branches, the bronchi, one of which enters each 

 king. 



Embedded in the tracheal connective tissue are the rings of 

 cartilage which keep this air passage dilated. In old animals they 

 may undergo partial ossification. The cartilages are not complete 

 rings, but usually have an open space between the ends adjacent 

 to the esophagus. This space between the ends of the cartilage is 

 filled by a band of smooth muscle and connective tissue. The 

 cartilages are enclosed in perichondrium and the superficial portions 



of the perichondrium merge with 



-Epithelium 



the adventitial connective tissues 



and with similar fibroelastic con- 



^i&'is^""''^''"""'"'"""'"" nective tissue l.ying between 



"^^^ Gland adjacent cartilages. The internal 



connective tissue may be called a 



-Submucosa submucosa and is rich in elastic 



fibers. In it are many small 



-Ptnchondrmm compound mucous and mixed 



^^::S.j^ s^-j^ 



^c^eD©^® mucoserous glands. ine sub- 



' Advent it ia 



«>@® ® '^to^J^^S"^'"''^^'^^^ mucosa also contains manv scat- 



„@ ^ '^ <s «®®® ecu , , 1 1 • , ' 



®i <s>%% t'S^^'^ tered lymphocytes which may 



^Ssgj ^%f^ ..s;£^ — Perichondrium be organized into lymph nodules. 



The mucosa is composed of 

 ciliated pseudostratified epithe- 



FiG 94 —Mammalian trachea cross- hum, in whicll are nUmerOUS 



^''*'*""^ goblet cells which add mucus 



to that from the submucosal glands, thus keeping the internal 

 surface of the trachea moistened. (Fig. 94.) 



The Lungs. — Since the respiratory system of manunals arose 

 embryologically as an invagination from the embryonic foregut, the 

 trachea represents the principal excretory duct which divides pos- 

 teriorly into two bronchi which branch into bronchioles. The lungs 

 are invested with a thin double membrane, the pleurie, similar in 

 structure to the peritoneum, with a potential space between the two 

 membranes. The pleural membrane adjacent to the lung contains 

 much smooth muscle in its connective tissue and di])s down into 

 the spaces between adjacent lung lobes to connect with the con- 

 nective tissue between the lobules. 



Bronchi and Branches. ^Each bronchus has about the same 

 structure as the trachea but the size of the tubes decrease as 



