PARENCHYMA OF THE LUNGS. 3G1 



the entire system of bronchial tubes, there are circular fasciculi 

 cf 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 -fa of an 

 inch in diameter. They 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 down to the air-cells. The 

 mucous membrane is smooth, covered by ciliated epithelium, 

 the movements of the cilise being always from within out- 

 ward, 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 aiid^ bronchi, racemose 

 glands exist in the membrane on the posterior surface of 

 the tubes ; but anteriorly are small follicles, terminating in 

 a single, and sometimes a double, blind extremity. These 

 follicles are lost in tubes less than -g 1 ^- of an inch in diameter. 

 It is the anatomy of the parenchyma of the lungs which 

 possesses the most physiological interest, for here the essential 

 processes of respiration take place. When moderately in- 

 flated, 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 thoracic walls, but are 

 closely applied to them, each covered by a reflection of the 

 serous membrane which lines the cavity on the corresponding 

 side. They thus necessarily follow the movements of ex- 

 pansion 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 divided into irregularly polygonal 

 spaces, from -J- of an inch to an inch in diameter, which mark 

 what are sometimes called the pulmonary lobules, though 

 this term is incorrect, as each of these divisions includes 

 quite a number of the true lobules. 



