ORGANS OF RESPIRATION. 387 



objects originating from blood corpuscles, which have been 

 arrested in their circulation through the minute vessels of 

 the structure of the air cells. So long as this retardation is 

 confined to the colourless corpuscles, the morbid actions 

 which ensue are strictly those of an abnormal nutrition, and 

 various forms of imperfect and degenerated epithelium are 

 the results ; but if it extend, so as to interfere with the free 

 circulation of the red corpuscles, we then have all the phe- 

 nomena of inflammation. No new elementary particles are 

 formed to constitute a tubercle, and although from the in- 

 sidious nature of the primary actions, from the delicacy of 

 the structure, and the important character of the function of 

 the lungs, the treatment of tubercular diseases must always 

 be attended with more than common difficulties, still there is 

 every reason to believe that they are susceptible of preven- 

 tion and cure, especially if our efforts for the attainment of 

 these ends be enforced previous to the appearance of a cough, 

 which is not a concomitant of the first stages of tubercular 

 deposition in the lungs." 



My own views of the nature of tubercle in the lungs differ 

 in one important respect from those of Dr. Addison ; that is, 

 with reference to the precise origin of the imperfect and de- 

 generated epithelial cells. Dr. Addison conceives that they 

 are derived immediately from the blood itself, while I con- 

 sider that they proceed from the epithelium, which has been 

 described as lining the air cells themselves. 



A tubercle then I would define as an accumulation of epi- 

 thelial scales, the imperfect and degenerate representatives of 

 the true epithelial cells of the organ or part in which the 

 tubercle is itself developed. 



A tubercle of the lungs, at the earliest period of its form- 

 ation, is exceedingly small, occupying a single cell only; 

 when this cell becomes filled with tubercular matter, a rup- 

 ture of its membrane occurs, and thus the extension of the 

 tubercle takes place from cell to cell; this extension at the 

 same time being accompanied by a destruction and displace- 

 ment of numerous of the capillary plexuses. 



After the above description, it need scarcely be observed 

 that tubercles of the lungs are not vascular. 



