6 PATHOLOGICAL MYCOLOGY. 



lower parts of the lung, the tuberculosis appears in the area in which, 

 owing to the obstruction of the bronchus, there is imperfect entrance 

 and exit of the air, in which, in consequence, there is an accumulation, 

 not only of what enters, but of what should normally escape 

 Should a so-called tubercle bacillus effect an entrance, it has every- 

 thing in its favour. It is not readily dislodged, it is at rest, it is sup- 

 plied with ample food stuff, it has an altered epithelial surface on 

 which to act, and it is on a surface between which and the neighbour- 

 ing lymphatics there is the closest communication. On the other 

 hand, in the respiratory tract, in addition to the above-mentioned 

 conditions, there is in the mouth a thick layer of squamous epithelium 

 which can withstand the attacks of the most vigorous bacillus (unless 

 it develops with extreme rapidity), and so act as a barrier against the 

 entrance of the bacillus into the lymphatics. In the lower parts of the 

 respiratory tubes the epithelial barrier consists of well-formed ciliated 

 columnar cells, the function of which is to pass on from one to another, 

 along with mucus and other material, the bacillus which is seeking 

 lodgment for long enough time to be able to develop morphologically 

 and physiologically, and invasion in this locality is prevented. We 

 shall return to the tubercle bacillus for further illustration and 

 comparison. 



Action on Tissues. 



5. If it be granted, for the sake of argument, that the germ theory of 

 disease is correct, in what way is it possible to account for the varied 

 changes, local or general, by which an organism manifests its presence ? 

 In certain cases we have an organism whose influence on the 

 ' system of its host is purely of a local nature, being itself circum- 

 scribed in its area of invasion, beyond which neither the organism 

 itself nor its products have any effect. Of such a class the soft 

 sore virus may be taken as a type. On the other hand, there may 

 be strict limitation of the organism to a definite area or tract, beyond 

 which it cannot penetrate, whilst its influence in the form of irritative 

 products may extend throughout the invaded organisation, often with 

 the gravest results. Such a class is represented in all probability by 

 the organisms present in cases of septicaemia. In a third class, 

 the organism is not limited in its sphere of activity, but penetrates 



