THE RESPIRATORY ORGANS. 



181 



The lung receives blood from two sources: 1, venous blood, 

 through the pulmonary artery, which is oxygenated in the walls of 

 the alveoli ; 2, arterial blood, through the bronchial arteries. This 

 arterial blood serves for the nourishment of the tissues of the lung 

 and is distributed to the bronchi, interlobular connective tissue, 

 lymph-glands, and walls of the vessels. Part of this blood returns 

 through the pulmonary veins; the rest through the bronchial veins. 



Fig. 157. 



a- 



Seetion of lung of the dog: a, oblique section of a bronchiole ; b, its muscular coat ; c, longi- 

 tudinal section of an infundibulum, communicating to the right with an alveolar passage 

 (the wall of the latter is torn further to the right) ; d, one of the alveoli opening into c. 



The lymphatics arise in the walls of the alveoli and bronchi and 

 pass to the bronchial lymph -glands. 



The nerves supplying the lung may be traced along the bronchi, 

 where they occasionally connect with groups of ganglion-cells, and 

 along the vessels. They are of both the medullated and the non- 

 medullated varieties. 



The surface of the lung is covered with serous membrane, a por- 

 tion of the pleura. 



Little need be said about the functional activity of the lung. 

 The cilia, belonging to the columnar epithelium lining nearly the 



