534 LYMPHATICS OF LUNG. [BOOK n. 



they pass from the left lung to the thoracic duct, and from the 

 right lung to the right lymphatic trunk. 



The impurities in the inspired air spoken of above as arrested 

 in the mucus lining the bronchial passages often make their way 

 through the epithelium into the lymphatics below and, carried 

 away in the lymph stream, are often retained in the bronchial 

 lymphatic glands. At times these glands become in this way 

 loaded with particles of carbon. 



The blood vessels of the lungs do not call for any special 

 comment save perhaps that the pulmonary veins are destitute 

 of valves ; and that special arteries, the bronchial arteries, starting 

 from the aorta, are distributed to the walls of the bronchial 

 passages, to the blood vessels, to the lymphatic glands and to the 

 sub-pleural tissue, the blood returning from them along the 

 bronchial veins into the right vena azygos on the right side, and 

 into the superior intercostal vein 011 the left side. 



323. The nerves to the lungs come chiefly from the vagus. 

 As, on each side, the vagus nerve winds round the root of the 

 lung, it gives off in front branches to form the anterior pulmonary 

 plexus, and then, behind, stouter branches to form the posterior 

 pulmonary plexus. Both these, but especially the latter, are 

 joined by filaments from the sympathetic system, more especially 

 from the second, third, and fourth thoracic ganglia; and it is 

 maintained by some that fibres pass direct from the spinal 

 (intercostal) nerves into these pulmonary plexuses. The upper 

 part of the trachea is supplied by twigs from the recurrent 

 laryngeal nerve on each side, and the lower part by twigs, 

 (tracheal branches) coming direct from the vagus trunks. 



Some of the nerve fibres thus reaching the lung along the 

 vagus nerve are efferent fibres for the muscular fibres of the 

 bronchial passages and trachea. But, as we shall see, the chief 

 and most important fibres are afferent fibres concerned in the 

 regulation of respiration. The functions of the fibres coming from 

 the sympathetic system have not yet been clearly ascertained ; 

 but there is evidence that some of the fibres coming from the 

 thoracic ganglia are vaso-motor (constrictor) fibres for the pulmo- 

 nary vessels. 



