100 PRACTICAL MICROSCOPY. 



A considerable amount of connective tissue accompanies and sup- 

 ports the organs which enter the lung, and is eventually in connec- 

 tion with the fibrous framework of the organ. 



The lung will, therefore, be seen to differ from organs generally, in 

 that it contains tiuo distinct: vascular supplies, viz., 1. The pulmo- 

 nary (of venous blood), entering for the purpose of its own oxygena- 

 tion; 2. The bronchial (arterial), which corresponds to the usual 

 nutrient blood supply of organs. 



THE PLEUKA. 



The lung is completely enveloped with a membrane composed ex- 

 ternally of pavement epithelium, 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 compartments or 

 lobules. The interlobular septa have usually become prominent in 

 the human adult from deposits of inhaled carbon in their lymph 

 channels. 



THE PULMONAKY ALVEOLI. 



The lung is constantly employed in maintaining the integrity of the 

 blood. This is accomplished by the exposure of the latter to a con- 

 tinual 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 the capil- 

 laries themselves, inasmuch as they are covered with a layer of flat 

 cells. These cells, constituting the parenchyma of the lung, have the 

 powder, on the one hand, of selecting such material from the air as 

 may be required, passing it on to the blood in the capillaries; and, on 

 the other, of removing effete materials from the blood, transferring it 

 to the atmospheric contents of the air sacs for exhalation. 



The air sacs or alveoli are not unlike minute bladders. Their dia- 

 meter about equals that of a terminal bronchus, viz., from one-one 

 hundredth to one-two hundredth of an inch. A group of these alve- 

 oli are associated in the manner shown in Eig. 72, their contiguous 

 walls fusing and all opening into a common cavity, the infundibulum. 

 The whole is in connection with a terminal bronchus vide (Eig. 73). 

 A primary lobule having been thus constructed, several are associ- 

 ated and united to a slightly larger bronchial twig, and there results 

 one of the polyhedral lobules, previously mentioned as visible, espe- 

 cially on the surface of the lung. By a repetiton of such elements 

 the lung is constructed. 



