304 



HISTOLOGY 



an ultimate lobule or structural unit. The veins are peripheral both in 

 the units and larger lobules; between the latter they run through connec- 

 tive tissue septa. 



The abundant lymphatic vessels are arranged in a superficial set drain- 

 ing into the pleura by way of the interlobular septa; and a deep set drain- 

 ing toward the hilus along the bronchi, accompanying the large vessels. 

 Lymphatics of the deep set do not extend into the lobules; they terminate 

 along the alveolar ducts. Around the larger bronchi and at the root of the 

 lung, lymph glands are numerous. A conspicuous feature of the sections 

 of the lung is the presence of black soot in the tissue around the lymphatic 



vein vessels. It penetrates the pulmo- 



nary epithelium in the smallest 



Capillaries. . , . . . 



bronchioles, apparently passing 

 Artery. between the epithelial cells. Some 

 of it is taken up by phagocytes. 

 Having entered the lymphatic 



FIG. 299. FROM A SECTION OF THB LUNG OF A VCSSels it IS distributed along their 

 CHILD, INJECTED THROUGH THE PULMONARY _ . e e . 



ARTERY, x so. courses. On the surface of the 



Of the five alveoli drawa^th^ three upper ones are lung ifc J s seen in the mte rlobular 



septa, marking out the boundaries 



of the lobules. Because of the steady increase in this deposit, the color 

 of the lungs changes from birth until old age. 



The nerves of the lung include the pulmonary plexus derived from the 

 sympathetic system. Its fibers enter at the root of the lung and spread 

 around the bronchi and vessels, to which they are chiefly distributed. 

 Small ganglia are found within these -plexuses. The vagus also sends 

 branches to the lungs, including medullated and non-medullated fibers, 

 which join the sympathetic plexuses. 



PLEURA. 



The visceral pleura is a thinner layer than the parietal pleura, and is 

 closely attached to the lung. It is covered with a single layer of flat 

 mesothelial cells, which in the collapsed lung become thicker and shorter. 

 In specimens which have been handled, this layer is often lacking. It rests 

 upon a thin layer of fine-meshed fibrous tissue, beneath which is the coarse 

 subserous layer continuous with the interlobular septa" of the lung (Fig. 

 301). This tissue is highly elastic. In the subserous layer, 'blood 

 vessels, derived from both pulmonary and bronchial arteries, form an 

 abundant capillary plexus. The superficial lymphatic vessels are very 

 evident, and in relation with them lymphoid tissue is found, and occa- 

 sionally lymph nodules. Stomata, which have been described, are pre- 

 sumably artificial apertures in the epithelium and are not connected with 

 the lymphatic vessels. 



