324 RESPIRATION 



phatic tissue surrounding bronchi and then pass right through 

 the walls of the bronchi and are swept out. Others reach the 

 lymphatic glands at the roots of the lungs, and finally seem to pass 

 from there into the blood. In this way the dust is removed from 

 the lungs, and if too much dust is not inhaled the process of re- 

 moval will keep pace with the introduction of dust. The well- 

 known "black spit" of a collier, which continues for long periods 

 when he is not working underground, is apparently a healthy 

 sign showing that dust particles are being removed from the 

 lungs. It seems quite probable, also, that the efficiency of the 

 physiological process for dealing with dust improves with use, 

 like other physiological processes. Moreover the dust-collecting 

 cells appear to be identical with cells which collect and deal 

 with bacteria in the lungs. Possibly, therefore, the somewhat re- 

 markable immunity of colliers from phthisis is connected with 

 their capacity for dealing with inhaled dust particles. 19 



At the end of a few months the lungs of a guinea pig which have 

 been heavily charged with coal dust or shale dust by experimental 

 inhalations are again free from dust. On the other hand this was 

 not the case when the dust inhaled was quartz. Most of the quartz 

 remained in situ, though mainly within the dust-collecting cells. 

 Part had, however, been carried onward to lymphatic glands. The 

 quartz did not seem to excite the cells to wander in the same way 

 as the coal dust or shale dust did ; and it appeared as if this dif- 

 ference in the properties of different kinds of dust explained why 

 some dusts are much more apt than others to produce cumulative 

 ill effects in the lungs. Presumably the quartz particles are so 

 inert physiologically that they do not excite the dust-collecting 

 cells to wander away. Other kinds of dust particles may be equally 

 insoluble, but may also be charged with adsorbed material which 

 makes them physiologically active. Coal, for instance, though very 

 insoluble in water, adsorbs substances of all kinds, and the im- 

 portance of its power of adsorbing gases has already been pointed 

 out. 



Shale dust was found by Dr. Mellor to contain about 35 per 

 cent of quartz. Nevertheless the quartz in shale dust does no harm 

 to the lungs and is eliminated readily. There are many other 

 kinds of stone which contain still more quartz, but also produce a 

 harmless dust. In fact nearly all the dust ordinarily met with is 

 of the harmless variety, and Mavrogardato's investigation indi- 



19 Haldane, Trans. Inst. of Mining Engineers, LV, p. 264, 1918. 



