574 VETEEINAKY HYGIENE 



An inherited tendency is very different from hereditary 

 transmission ; there are certain animals, rodents for 

 example, where the inherited tendency is very marked, they 

 can rapidly be made tubercular, for their tissues afford a 

 favourable soil, but the disease is not the result of hereditary 

 transmission. 



A clear understanding of this fact exercises the most im- 

 portant influence in the hygiene and treatment of tubercu- 

 losis ; if animals are not born tubercular but are made so, 

 then the question of control is simplified. What the 

 hygienist has to consider is how animals are made tuber- 

 cular, and when this is understood control can be exercised. 



The destructive lung lesions when sufficiently advanced 

 cause coughing and the ejection of sputum. Microscopical 

 examination and experimental inquiry have placed beyond 

 all shadow of doubt that the sputum of a tubercular patient 

 contains the bacilli of the disease. 



Every act of coughing therefore ejects into the air of a 

 building a certain number of organisms, of which it is per- 

 missible to believe all do not survive. It is quite certain 

 that the bacilli once ejected cannot multiply outside the 

 body, for a constant temperature near that of the animal 

 body is required for this purpose. These bacilli will there- 

 fore in course of time die, but their death is not immediate ; 

 in the first instance they are fixed in the moist expectorate, 

 but later on the sputum dries, and in this condition the still 

 living tubercle bacilli rise with the dust in the air, by which 

 means they find their way again into the lungs by inhala- 

 tion. 



This simple explanation of infection is of the utmost im- 

 portance to the hygienist ; a tubercular animal in the midst 

 of healthy is a focus of infection, and it is obvious that the 

 probability of infection is infinitely greater in an enclosed 

 space like a cow-shed than in the open air. In fact, the 

 risk of infection in the open can hardly be measured, for 

 the evidence on that point is insufficient. 



Infection in closed spaces is intensified by a stagnant 

 atmosphere, such as in a badly ventilated byre, by over- 



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