PLEURA. 243 



manent apertures (stomata) in the epithelium probably do not exist. The 

 connective tissue of the pulmonary pleura contains many elastic fibers; 

 these are less abundant in the parietal pleura. Fat is found, sometimes 

 forming folds (plicae adiposae] and the vascular elevations suggestive of 

 synovial villi are called pleural mill. These may be sought toward the 

 median wall, beneath the lung. The nerves of the pleura, derived from 

 the phrenic, sympathetic and vagus are said to possess small ganglia. In 

 the parietal pleura typical lamellar corpuscles and some of their varieties 

 (Golgi-Mazzoni corpuscles) have been found. The blood vessels of the 

 pleura are said to include branches both of the pulmonary and the bron- 

 chial vessels. Lymphatic vessels are numerous and small lymph glands 

 occur. 



Septa extend from the pleura into the lung thus dividing its super- 

 ficial portion into lobules from i to 3 cms. in diameter. They are visible 

 on the surface as polygonal areas bounded by pigmented lines. Since 

 these lobules consist of smaller subdivisions also called lobules, the former 

 are designated as secondary and the latter as primary lobules (structural 

 units). 



In the connective tissue between the secondary or larger lobules, 

 lymphatic vessels make their way to the pleura and thence over the surface 

 of the lung to its root. These lymphatic vessels constitute the superficial 

 system. The deep lymphatic vessels begin along the small bronchioles 

 and the adjoining vessels, and they accompany the arteries, veins, and 

 bronchi to the root of the lung. To some extent the superficial and deep 

 systems communicate. No lymphatic vessels are found beyond the alveo- 

 lar ducts, within the lobules. Along the larger bronchi and toward the 

 root of the lung lymph glands are numerous. 



Black pigment is generally abundant along the course of the lymphatic 

 vessels. It is not melanin but soot, which is absent from the lungs at birth 

 but accumulates with age, especially in certain environments. It pene- 

 trates the pulmonary epithelium chiefly in the smallest bronchioles, ap- 

 parently passing between the cells. Some of it is taken up by phagocytes. 

 Having entered the lymphatic vessels it becomes distributed along their 

 courses. 



The blood vessels accompany the bronchi. In the primary or ultimate 

 lobules the arteries are central, producing a terminal branch for each 

 atrium or alveolar sac (Fig. 271). The veins arising from the alveolar 

 capillaries pass over the peripheral surface of the structural units as shown 

 in the figure. The distribution of the bronchial vessels has already been 

 noted. 



The nerves of the lung include a pulmonary plexus from the sympa- 



