128 STUDENTS HISTOLOGY 



THE PULMONARY BLOOD-VESSELS 



The prominent accompaniments of the bronchus, at the root of 

 the lung, are the pulmonary artery (carrying venous blood) and 

 the pulmonary veins. 



The pulmonary artery enters the lung with the bronchus, fol- 

 lowing its ramifications, to end in capillary plexuses in the walls of 

 the sac -like dilatations, which are in connection with the ultimate 

 bronchial tubes. The blood is then collected in venules, which 

 unite to form the pulmonary veins. The latter pursue an inde- 

 pendent course in their exit, not accompanying the bronchial tubes 

 until the large bronchial tubes have been reached. 



The bronchial artery (nutrient) enters with the bronchus, sup- 

 plying its walls and the connective tissue framework of the lung. 



A considerable amount of connective tissue accompanies and 

 supports the structures which enter the lung, and is eventually in 

 connection with the fibrous framework of the organ. 



The lung will, therefore, be seen to differ from organs gener- 

 ally, in that it contains two distinct vascular supplies, viz. : (1) The 

 pulmonary (of venous blood), entering for the purpose of its own 

 oxygenatiou; (2) The bronchial (arterial), which corresponds to 

 the usual nutrient blood -supply of organs. 



THE PLEUEA 



The lung is completely enveloped in a membrane composed 

 externally of endothelium, while the visceral portion is made up of 

 interlacing fibrous and elastic tissue. The deep or visceral layer 

 of the pleura sends prolongations in the form of septa into the 

 substance of the lung, dividing it into rounded polyhedral compart- 

 ments or lobules. The interlobular septa have usually become 

 prominent in the human adult from deposits of inhaled carbon in 

 their lymph -channels. 



THE PULMONARY ALVEOLI 



The lung is constantly employed in maintaining the integrity 

 of the blood. This is accomplished by the exposure of the latter to 

 a continual supply of atmospheric air. The air is introduced into 

 little sacs (termed air-vesicles or alveoli), in the walls of which the 

 blood is distributed in a capillary plexus. The air does not reach 



