THE RESPIRATORY TRACT. 265 



The nerves of the lungs are derived from the sympathetic 

 and pneumogastric. Their mode of termination is not known. 



For the examination of the general structure of the lung it 

 may be inflated and dried pretty rapidly in the sun or by a 

 fire. For more careful examination it should be hardened in 

 chromic acid, Miiller's fluid, or alcohol. The hardening fluid 

 should be injected into the air- passages. 



In order to distend the vesicles it is well, before placing the 

 lung iri the hardening fluid, to inject the bronchi with simple 

 gelatine. The vessels may also be injected with a colored 

 mass. The lungs of the lower animals are used for these 

 demonstrations, owing to the difficulty of obtaining normal 

 human lungs in a perfectly fresh condition. The investigation 

 of the lymphatics is attended with great difficulty. They may 

 be demonstrated by the puncture method. Klein found that 

 on injecting the blood-vessels, under high pressure, with Ber- 

 lin blue or silver nitrate, some of the capillaries ruptured, and 

 the fluid passed into the perivascular lymphatics. 



The pleura. The pleura, like the other serous membranes, 

 consists of a connective-tissue ground-substance covered by a 

 single layer of polygonal endothelial cells. In the costal pleura 

 the subserous connective tissue is more abundant, and its at- 

 tachment to the thoracic wall is not so firm as is that of the 

 pulmonary pleura to the lung. The structure of the pleura is 

 most conveniently studied in the smaller mammals. It can 

 also be demonstrated in young children. 



To demonstrate the endothelium of the surface, the thorax 

 of a recently killed animal should be opened, care being taken 

 not to rub or otherwise injure the pleura. The surfaces are to 

 be washed by pouring distilled water over them, in order to 

 remove the serum, and then a weak solution of silver nitrate 

 (4 I per cent.) allowed to flow over them. After a few mo- 

 ments the surfaces are bathed with pure water. The diaphragm- 

 atic or mediastinal portion is then excised with scissors, 

 immersed in distilled water or glycerine, and exposed to the 

 daylight until it takes a light reddish-brown color. It may 

 now be floated on to a slide, carefully smoothed by traction at 

 the edges, and mounted in glycerine. The portion excised 

 should be large, so that it can be manipulated without touch- 

 ing the part which is to be examined. For this reason it is well 

 to take with it some of the surrounding structures, e.g., the 



