PHYSIOLOGICAL ANATOMY OF THE RESPIRATORY ORGANS. 119 



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

 branes ; an external membrane, between the layers of which the cartilages 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 number of unstriped, or involuntary 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. This collection of muscular fibres is sometimes 

 called the trachealis muscle. Throughout the entire system of bronchial tubes, there 

 are circular fasciculi of muscular fibres lying just beneath the mucous membrane, with a 

 number of longitudinal elastic fibres. The character of the bronchi abruptly changes 

 in tubes less than ^ of an inch in diameter. They lose 4 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 down to the air- 

 cells. The mucous membrane is smooth, covered by ciliated epithelium, the movements 

 of the cilia being always from within outward, and it is provided with numerous mucous 

 glands. These glands are of the racemose variety and, in the larynx, are of considerable 

 size. In the trachea and bronchi, racemose glands exist in the membrane on the posterior 

 surface of the tubes ; but anteriorly are small follicles, terminating in a single, and some- 

 times a double, blind extremity. These follicles are lost in tubes measuring less than ^ 

 of an inch in diameter. 



FIG. 87. Bronchia and lungs, posterior view. (Sappey.) 



1, 1, summit of the lunQs; 2, 2, base of the lung* ; 8, trachea ; 4. right bronchus ; 5, diriaion to the upper lob* 

 of the lung; 0, dirlnion to the lower lobe ; 7, left bronchus ; 3. division to the upper Job, ; '.), ilirlxiiin to the. 

 lower lobe; 10, left branch of the pulmonary artery : 11. risrht braiich ; 12, left auricle of the heart: \A. K-ft MI- 

 perior pulmonary vein ; 14, left inferior pulmonary vein ; 15. ritrht superior pulmonary vein ; 1(5, right inferior pul- 

 monary vein; 17, inferior vena cava; IS, left ventricle of the heait; ID, right ventricle. 



It is the anatomy of the parenchyma of the lungs which possesses the most physio- 

 logical interest, for here the essential processes of respiration take place. When mod- 

 erately inflated, the lungs have the appearance of irregular cones, with rounded apices, 

 and concave bases resting upon the diaphragm. They fill all of the cavity of the chest 

 which is not occupied by the heart and great vessels, and are completely separated from 

 each other by the mediastinum. In the human subject, the lungs are not attached to the 



