124 THE ESSENTIALS OF HISTOLOGY 



serous membrane (subserous tissue). In some animals tbis subserous 

 layer contains plain muscular tissue, whicb is especially developed 

 near the lung-apex. 



The lymphatics of tbe lung form two sets of vessels, one set 

 accompanying 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 hilus 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 mem- 

 brane with the cavity of the pleura. 



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

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

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

 bronchial arteries. 



