THE LUNGS. 



171 



most usually evident and the more so, because their boundaries 

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



Fig. 237. 



of time, is deposited in the continuous interlobular, connective 

 tissue. These lobules are ultimately united together by a more 

 abundant interstitial 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. 



§ 177. 



The intimate structure of the bronchia and air-cells is as 

 follows. The bronchia are in general constituted in the same 

 way as the air-tubes and their branches, although from the 

 very commencement some differences are presented, which be- 

 come greater and greater in their further course. It is most 

 proper to distinguish in them two membranes, a fibrous, still 

 in part containing cartilages, and a mucous, with a smooth 

 muscular layer. The former, constituted of connective tissue 

 and elastic fibrils, is at first thick, as in the bronchi, but gra- 

 dually becomes thinner and thinner ; in bronchia, less than 

 $"' in diameter, it is scarcely demonstrable with the scalpel; 



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

 wax, x 30 diam., after Harting : a a t air-cells ; b b, borders of the smallest lobules 

 or infundibula (Rossignol). 



