68 Causes of Disease. 



effect of a variet\ of agencies. A nasal catarrh may be caused 

 by the inhalation of (hist, irritant gases, chemical llnids, the 

 influence of cold upon the external surface of the body, or from 

 infection: cold may produce in one subject painful peristalsis 

 (colic), in another muscular rheumatism, in a third a coryza, a 

 pneumonia, an intestinal catarrh. In infectious diseases the body 

 generally reacts with uniformity in a definite manner ; the basic 

 symptoms and lesions are invariably manifest, and it is possible 

 to decide from the presence of certain signs of the disease that a 

 definite type of infection exists. By specificity, therefore, we 

 understand that each infectious disease is the result of fixed in- 

 fectious agents and not of anything else. Anthrax is always 

 caused by the anthrax bacillus, and never by any other microbe 

 or noxious agency of any sort ; glanders is caused by the glanders 

 bacillus ; smallpox by the smallpox contagium. There are, how^- 

 ever, some micro-organisms which, because of similarity of toxic 

 properties and analogous modes of multiplication, give rise to the 

 same anatomical tissue changes, so that clinically the same 

 anatomical types of disease are produced by several kinds of 

 germs (the groups of the pyogenic bacteria, the micro-organisms 

 of septicaemia, of moist gangrene and of mastitis). Such in- 

 fectious diseases are said to be polybacterial. 



Variations in flic duration of infectious discuses, modifications 

 and degrees of anatomical changes, ore proportioned in the sei'cr- 

 ity of the infection to the grade of virulence of the microbes, 

 and are dependent as well upon the place of entrance of the infec- 

 tion and the predisposition of the tissues. Just as a corrosive 

 substance, an acid, depending upon whether it comes in contact 

 with animal tissue in concentrated or dilute form, produces either 

 an eschar, tissue necrosis, inflammation or mere hyperaemia. so a 

 difference is appreciable in the action of microbes according as 

 they are highly toxic or more or less attenuated. The importance 

 of the factor exerted by the place of entrance of the infection is 

 indicated by the studies of Arloing, Cornevin and Thomas upon 

 the bacillus of symptomatic anthrax ("black leg") ; this micro- 

 organism, if inoculated into the muscles and subcutaneously. kills 

 animals after intense symptoms ; but if the inoculation be made 

 into the trachea or intravenously there follows merely an abortive 

 course, which, however, is succeeded by an immunity. \'ariations of 

 susceptibility and resistive powers in different animal species and 

 individuals to precisely the same infectious agent may cause 



