THE LUNGS. 



577 



injected, in consequence of their mutual pressure, rounded-angular ; the 

 air-cells of the surface of the lung are invariably polygonal and their 

 external sides are almost always nearly plane. 



The lohulated strucUire of the lung is not nearly so distinct in the 

 human adult as in younger individuals and in animals. It is therefore 

 advisable, in the first instance, to seek for these conditions in the luntrs 

 of a child. In this case the separate lobules are still all distinctly 

 parted from each other by connective tissue and admit of being isolated, 

 so that the tolerably regular pyramidal form of the superficial, and the 

 more irregular one of the interior lobules can be satisfactorily perceived. 

 In the adult, also, these smallest lobules, in size -i-|-l line, still exist, 

 but are so intimately united, that, even on the surface of the lung, their 

 outlines are only 'perceived with difficulty and imperfectly ; and, in the 



Fiff. 237. 



interior of the organ, a more homogeneous structure, something like 

 that of the liver, is apparently presented. Secondary lobules, on the 

 other hand, of 5-J-I inch in size (lobules of authors) are, even in the 

 adult, most usually evident and the more so, because their boundaries 

 are indicated by streaks of pigmentary matter, which, in course of 

 time, is deposited in the continuous interlobular, connective tissue. 

 These lobules are ultimately united together by a more abundant inter- 

 stitial tissue, so as to form the large well-known lobes. 



Thus the lung consists entirely of larger and smaller groups of air- 

 cells and smallest bronchial tubes, and accordingly the larger air-vessels 

 also fall into certain definite groups, each of which stands in relation 

 with only one of the former. 



Fig. 237. — External surface of the lung of a Cow, with the air-cells injected with wax, 

 magnified 30 diameters; after Harting: a a, air-cells; 6 6, borders of the smallest lobules or 

 infundibula (Rossignol). 



37 



