ANATOMY OF THE EESPIKATORY ORGANS. 



113 



which is external. The collection of muscular fibres in the posterior part of 

 the trachea is sometimes called the trachealis muscle. Throughout the 

 bronchial tubes, 

 there are circular 

 fasciculi of non- 

 striated muscular 

 fibres lying just 

 beneath the mu- 

 cous membrane, 

 with a number of 

 longitudinal elas- 

 tic fibres. The 

 character of the 

 bronchia abruptly 

 changes in tubes 

 less than -^ of an 

 inch (O5 mm.) in 

 diameter. They 

 then lose the car- 

 tilaginous rings, 

 and the external 

 and the mucous 

 membranes be- 

 come so closely 

 united that they 

 can no longer be 

 separated by dis- 

 section. The cir- 

 cular muscular fibres continue as far as the air-cells. The mucous mem- 

 brane is smooth, covered by ciliated epithelium, the movements of the cilia 

 being always from within outward, and it is provided with mucous glands. 

 These glands are of the racemose variety and in the larynx they are of con- 

 siderable size. In the trachea and bronchia, racemose glands exist in the 

 membrane on the posterior surface of the tubes ; but anteriorly are small fol- 

 licles, terminating in a single, and sometimes a double, blind extremity. 

 These follicles are lost in tubes measuring less than -fo of an inch (0'5 mm.) 

 in diameter. 



When moderately inflated, the lungs have the appearance of irregular 

 cones, with rounded apices, and concave bases resting upon the diaphragm. 

 They fill that part of the cavity of the thorax which is not occupied by the 

 heart and great vessels, and are completely separated from each other by the 

 mediastinum. The lungs are in contact with the thoracic walls, each lung 

 being covered by a reflection of the serous membrane which lines the cavity 

 of the corresponding side. Thus they necessarily follow the movements of 

 expansion and contraction of the thorax. Deep fissures divide the right lung 

 into three lobes and the left lung into two. The surface of the lungs is di- 



FIG. 41. Bronchia and lungs, posterior view (Sappey). 



1,1, summit of the lungs ; 2, 2, base of the lungs ; 3, trachea ; 4, right bronchus; 

 5, division to the upper lobe of the lung ; 6, division to the lower lobe ; 7, 

 left bronchus ; 8, division to the upper lobe ; 9, division to the lower lobe ; 

 10, left branch of the pulmonary artery; 11, right branch; 12, left auricle 

 of the heart ; 13, left superior pulmonary vein ; 14, left inferior pulmonary 

 vein ; 15, right superior pulmonary vein ; 16, right inferior pulmonary 

 vein ; IT, inferior vena cava ; 18, left ventricle of the heart ; 19, right ven- 

 tricle. 



