94 RESPIRATION 



In the trachea and bronchia, racemose glands exist in the membrane on 

 the posterior portion of the tubes ; but anteriorly are small follicles, ter- 

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

 follicles are lost in tubes measuring less than -fa of an inch (0.5 milli- 

 meter) in diameter (see Plate II, Fig. 4). 



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 with a reflection of the serous membrane that 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 divided into irregularly polygonal spaces, ^ of an 

 inch to an inch (6.4 to 25.4 millimeters) in diameter, which mark what 

 are sometimes called the pulmonary lobules ; but this term is incorrect, 

 as each of these divisions includes a number of the true lobules. 



Following out the bronchial tubes from the diameter of -g 1 ^ of an inch 

 (0.5 millimeter), the smallest, which are -j| 7 to y 1 ^ of an inch (0.21 to 0.33 

 millimeter) in diameter, open into a collection of oblong vesicles, which are 

 the air-cells. Each collection of vesicles constitutes one of the true pul- 

 monary lobules and is -fa to ^ of an inch (0.5 to 2.1 millimeters) in 

 diameter. After entering the lobule, the tube forms a tortuous central 

 canal, sending off branches which terminate in groups of eight to fifteen 

 pulmonary cells, or alveoli. The cells are a little deeper than they are 

 wide and have each a rounded blind extremity. Some are smooth, but 

 many are marked by little circular constrictions. In the normal lung of 

 the adult, after death, they measure %fo to T -|- - or ^ of an inch (0.125 to 

 0.21 or 0.36 millimeter) in diameter, but are capable of great distention. 

 The smallest cells are in the deep portions of the lungs, and the largest 

 are near the surface. There are considerable variations in the size of the 

 cells at different periods of life. The smallest cells are found in young 

 children, and they progressively increase in size with age. The walls 

 of the air-cells contain abundant small elastic fibres, which do not form 

 distinct bundles for each air-cell, but anastomose freely with each other, 

 so that the same fibres belong to two or more cells. This structure is 

 peculiar to the lungs and gives to these organs their great distensibility 

 and elasticity, properties which play an important part in expelling the 

 air from the chest, as a consequence simply of cessation of the action of 

 the inspiratory muscles. Interwoven with these elastic fibres, is the 

 richest plexus of capillary bloodvessels found in the economy. The 



