THE RESPIRATORY ORGANS. 255 



nary artery follow the air-tubes to their ultimate distribution, the 

 arterioles extending along the respiratory bronchial tubes and terminal 

 ducts as far as the interatrial septa. They there end in capil- 

 lary net-works which surround the air-sacs in the manner above 

 described. In their course along the respiratory bronchial tubes and 

 the terminal ducts the pulmonary arterioles give off twigs which form 

 net-works around the air-cells besetting those passages. The 

 blood of the alveolar net-works is carried away by the radicles of the 

 pulmonary veins, which begin at the margins of the air-cells. 



In addition to the system of vessels derived from the pulmonary 

 artery destined for the respiratory function, a second group, for 

 the nutrition of the pulmonary tissues, is distributed by the 

 bronchial arteries. These vessels run in company with the bron- 

 chial tubes and the other blood-vessels within the interlobular con- 

 nective tissue and give off twigs which break up into the capillaries 

 immediately supplying the walls of the air-passages and associated 

 structures. Additional capillaries supply the interlobular areolar 

 tissue and the pleural tissues on the surface of the lungs. 



The numerous lymphatics of the lung are arranged as two sets, 

 those originating within the connective-tissue septa and those 

 arising in connection with the bronchial mucous membranes. 

 Of the former two groups are recognized, one of which includes the 

 channels beginning within the interlobular fibrous tissue and 

 forming the lymphatics which accompany the branches of the pul- 

 monary blood-vessels ; the other, the superficial set, arises by the 

 radicles connected with the subpleural lymph-spaces, which 

 communicate with the serous cavity of the pleura by means of the 

 minute passages leading from the intercellular orifices of the pleural 

 surface into the subjacent lymph-clefts. 



The bronchial lymphatics originate within the subepithelial 

 lymph-spaces which communicate with the mucous surfaces of the 

 air-tubes and the alveoli through the stomata; from the subepithelial 

 plexus larger lymph-channels unite with others to form definite lym- 

 phatic canals ; these accompany the blood-vessels to the root of the 

 lung, where the superficial and deep lymphatics meet and are taken 

 up by a few trunks of large size which pass from the lung to the 

 bronchial lymph-glands. Masses of lymphoid tissue of varying 

 extent are associated with the walls of the alveolar ducts and the 

 bronchial tubes, as well as the subpleural and peribronchial areolar 

 tissue. 



The nerves of the lung include contributions from both the cerebro- 

 spinal and the sympathetic system. The nerve-trunks, made up 

 of medullated and pale fibres, enter the organ at its root and follow 

 the air-tubes and the blood-vessels. Small groups of ganglion-cells 



