MEANS OF INGRESS. 5 



* 

 present only in inconsiderable numbers, may make their appearance in 



enormous masses, often to the exclusion of those usually found. This 

 is especially the case where, in addition to altered secretion, there is 

 imperfect removal of excreta. Hence even mechanical conditions 

 may have considerable influence in determining the reactions of the 

 tissues and the micro-organisms upon one another, as constrictions, 

 pouches, cavities, and wounds, internal or external, may, any of them, 

 give rise to an accumulation of fluids or excreta which act, firstly, by 

 weakening the tissues, and, secondly, by affording nidus and pabulum 

 to organisms, which, without such a resting-place, could find no base 

 of operations even upon the weakened tissues. The importance of 

 the structural peculiarities of a part in relation to its power of resis- 

 tance against the attack of micro-organisms, can only be fully appre- 

 ciated after a careful consideration of these peculiarities. The 

 structure, function, and arrangement of the epithelium covering the 

 surface, the position of the surface, and its relations, first, to the 

 external world, and, second, to the deeper tissues, especially to the 

 lymphatics, the number, size and relations of these lymphatics, the 

 tissues in which they occur, the density, vascularity and vitality of 

 these tissues, and the relations of the lymphatics to the small veins, 

 are, amongst others, points to which special attention must be paid. 



If tuberculosis be regarded as a specific infective disease, due 

 to the presence and activity of a certain minute organism, how is it 

 that certain individuals are attacked whilst others escape, that in those 

 attacked the organ most frequently assailed is the lung, and that the 

 selective process passes even further, and that th^ apex of the lung 

 is specially affected ? We can readily understand that the mouth, 

 fauces, trachea, and air passages generally are not affected, because 

 there is a continual emptying of these cavities, a constant ebb and 

 flow of air, and also of the materials which this air carries with it, 

 keeping up a state of unrest — a state which is fatal to the growth of 

 most micro-organisms. This tidal change extends to all parts of the 

 lung where there are proper expansion and contraction, which, as 

 pointed out by the older physicians, fails first at the apex, the position 

 in which phthisis commences. In children, where there has been 

 great enlargement of a bronchial gland, or where, from any other 

 cause, there has been pressure upon a bronchus passing to one of the 



