PHYSIOLOGICAL ANATOMY OF THE RESPIRATORY ORGANS 93 



the cartilages are situated, and a mucous membrane. The external mem- 

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

 the space not covered with cartilaginous rings, these fibres are mixed 

 with a certain number 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, which is external. The collection of mus- 

 cular fibres in the posterior part of the trachea is sometimes called the 

 trachealis muscle. Throughout the bronchial tubes, there are circular 



Fig. 30. Bronchia and lungs , posterior view (Sappey),. 



I, i, 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; n, 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 ; 17, inferior vena cava ; 18, left ventricle of the 

 heart; 19, right ventricle. 



fasciculi of non-striated muscular fibres lying just beneath the mucous 

 membrane, with a number of longitudinal elastic fibres. The character 

 of the bronchia abruptly changes in tubes less than -$ of an inch (0.5 milli- 

 meter) in diameter. They then lose the cartilaginous rings, and the 

 external and the mucous membranes become so closely united that they 

 can no longer be separated by dissection. The circular muscular fibres 

 continue as far as the air-cells. The mucous membrane is smooth, 

 covered with ciliated epithelium, the movements of the cilia being from 

 within outward, and is provided with mucous glands. These glands are 

 of the racemose variety, and in the larynx they are of considerable size. 





