112 



RESPIRATION RESPIRATORY MOVEMENTS. 



lungs, divide and subdivide, until the minute ramifications of the bronchial 

 tree open directly into the air-cells. After penetrating the lungs, the carti- 



FIG. 40. Lungs, anterior view (Sappey). 



1, upper lobe of the left lung ; 2, lower lobe : 3. fissure ; 4. notch corresponding to the apex of the heart ; 

 5, pericardium : 6, upper lobe of the right lung ; 7, middle lobe ; 8, lower lobe ; 9, fissure ; 10, fissure ; 

 11, diaphragm : 12, anterior mediastinum ; 13, thyroid gland ; 14, middle cervical aponeurosis ; 15, 

 process of attachment of the mediastinum to the pericardium ; 16, 16, seventh ribs; 17, 17, transver- 

 sales muscles ; 18, linea alba. 



lages become irregular and are in the form of oblong, angular plates, which 

 are so disposed as to completely encircle the tubes. In tubes of very small 

 size, these plates are fewer than in the larger bronchia, until, in tubes of a 

 less diameter than ^ of an inch (O5 mm.), they disappear. 



The walls of the trachea and bronchial tubes are composed of two distinct 

 membranes ; an external membrane, between the layers of which the carti- 

 lages are situated, and a lining, mucous membrane. The external membrane 

 is composed of inelastic and elastic fibrous tissue. Posteriorly, in the space 

 not covered by cartilaginous rings, these fibres are mixed with a certain num- 

 ber of non-striated muscular fibres, which exist in two layers ; a thick, internal 

 layer, in which the fibres are transverse, and a thinner, longitudinal layer, 



