148 THE ESSENTIALS OF HISTOLOGY. 



within the alveoli. The capillary network of the alveoli is very 

 close (fig. 171), and the capillary vessels of adjoining alveoli are in 

 complete continuity, the vessels passing first to one side and then to 

 the other of the septa which separate the adjacent alveoli. 



Blood-vessels. Branches of the pulmonary artery accompany the 

 bronchial tubes to be distributed to the capillary networks upon the 

 alveoli, from which the blood is returned by the pulmonary veins. 

 These, pursuing a separate course through the tissue of the lung, join 

 in their course with others to form larger vessels which pass to the 

 Jiilum. Branches from the bronchial arteries are distributed to the 

 walls of the bronchial tubes, and to the connective tissue of the lung. 

 This tissue intervenes everywhere in small quantity between the 

 infundibula (interstitial tissue), and forms a distinct layer, containing 

 much elastic tissue, covering the surface of the lung underneath the 

 serous membrane (subserous tissue.) In some animals (e.g. guinea-pig) 

 this subserous layer contains plain muscular tissue, which is especially 

 developed near the lung-apex, but it has not been detected in man. 



The lymphatics of the lung form two sets of vessels, one set accom- 

 panying the bronchial tubes, and another set forming a network in the 

 interstitial connective tissue, and in the subserous tissue. Both sets of 

 lymphatics tend towards the hilum, and enter lymphatic glands at the 

 root of the lung. Those in the subserous tissue communicate by means 

 of stomata between the epithelial cells of the serous membrane with the 

 cavity of the pleura. 



The pleura, which covers the surface of the lung, has the usual struc- 

 ture of a serous membrane. It is provided with a special network of 

 capillary blood-vessels, which are supplied by branches of the bronchial 

 arteries. 



