l6o FIFTEENTH LECTURE. 



The primary pulmonary lobules of the new-born — later the 

 nature of the arrangement becomes more indistinct — united 

 by connective-tissue intermediate substance, form larger or 

 secondary lobules. The latter appear on the surface of the 

 organ in the human adult as areae. measuring I to 2 mm. and 

 more, demarcated by a black substance, and often appearing 

 quite distinct. They form, at last, the large lobes. Their 

 delineation belongs to descriptive anatomy. 



We have just mentioned the black substance in the inter- 

 lobular connective tissue ; it may occur between and in the 

 walls of the pulmonary vesicles, and even in the bodies of 

 their epithelial cells, as we shall mention hereafter. This is 

 the so-called black lung pigment. 



We have just used the epithet "so-called." In fact these 

 substances are not melanine, the complicated, dark ferrugi- 

 nous coloring matter of the organism. They have rather an 

 extraneous origin ; they are carbon, breathed in in a finely 

 divided condition, which is induced by our artificial life in 

 enclosed place's. 



Mammals living wild show nothing of this, but it is seen 

 in their kin when domesticated by man. In human beings 

 constantly surrounded by smoke and soot, or in laborers in 

 coal mines, the lungs may at last become quite black. If we 

 shut a dog up in a place in which there is a constant genera- 

 tion of soot, a similar change of the respiratory organs takes 

 place with relative rapidity. 



In a condition of the finest division, these particles of car- 

 bon penetrate the epithelial cells, and from them enter the 

 pulmonary tissue. A great portion of them here become 

 permanently quiet. Others enter the lymphatics, and pass 

 from these into the lymphoid bronchial glands. They also 

 become fixed in the latter organs. This is the so-called mela- 

 nosis of these structures. 



Let us now examine the vascular arrangement. 



By the continual division of the pulmonary artery, there 

 arises a system of fine blood-vessels, which encircle the in- 

 dividual pulmonary vesicles, and frequently combine into 



