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indispensable to the healthy performance of the respiratory 

 function. As in the interraittents from marshy emanations, 

 so in typhus, we have a well defined limit in general to its 

 extension, as though the power of the poison had been 

 impaired or even lost by diffusion; and that periodically 

 recurring epidemics, the influenza excepted, should almost 

 invariably select their victims from the same classes of 

 persons, and from the same neglected parts of populous 

 towns, sufficiently demonstrates the intimate connexion that 

 exists between disease and surrounding impurities. As 

 already observed when considering the gases, a distinction 

 should be drawn between the malarious corpuscles which I 

 not only consider to exist, though certainly not endowed 

 with organic life, but to be disseminated by the aid of 

 aqueous vapour, and the simple exhalations of odorous par- 

 ticles. The former I believe to be absorbed, chiefly through 

 the pulmonary tissue, into the circulation, which with Liebig 

 I consider the part of the human organism most vulnerable 

 to exterior influences; whilst the latter effect their access 

 to the body mainly by instantaneous impressions on the 

 olfactory and nervous expansions generally of the mucous 

 membranes. The united operation of these agents of in- 

 fection I have often seen induce immediate illness ; whereas 

 the single action of the first commonly affords an interval 

 or latent period previous to the full development of the 

 poisonous effects engendered by it. In the first case we 

 have what has been termed the organic ferment, whilst in the 

 last the blood does not necessarily undergo change. 



I have thus deemed it desirable we should glance at the 

 nature of infecting materials and the channel whereby they 

 may enter to contaminate the system, because on a tolerably 

 accurate knowledge of these points must be founded all 

 rational means of correction. Various, however, as are the 

 sources of contamination, and whatever views may be held 

 as to the precise mode of action in the process of infection, 



