664 MICROBIOLOGY OF DISEASES OF MAN AND DOMESTIC ANIMALS 



one authority for the statement that the internal organs of healthy 

 animals are sterile. This has been shown not to be the case universally. 

 Experiments have shown that fifty per cent of the internal organs of 

 rabbits, guinea-pigs, cats, dogs, mice, horses and cattle are not sterile. 

 Bact. tuberculosis has been found in absolutely normal human and 

 bovine . lymph glands. The various pus-producing micrococci have 

 been frequently found in the spleen, kidney, liver, etc. Perhaps the 

 commonest group of bacteria to be isolated ff om the internal organs are 

 the intestinal forms. It has been demonstrated that intestinal micro- 

 organisms invade the tissues with surprising rapidity when for any 

 reason the resistance of the body is lowered. It has been noted also 

 that there are more bacteria in the internal organs of animals which 

 have been fasted than in those which have been fed. Peristaltic action 

 and the diffusion of food through the intestinal wall may be influencing 

 factors. The fact that the internal organs are not sterile in every case 

 is important as it may account for the so-caUed autogenic infections. 



The Manner Infectious Agents Enter the Body and Their 



Source f 



Air-borne Infections. — The ' causal microorganisms of infectious 

 diseases are frequently excreted from the body of the diseased individual 

 and are deposited on the clothing, furnishings, on the floors and walls, 

 or on the ground. These microorganisms probably do not proliferate 

 except in rare instances, but frequently remain virulent for a short 

 period of time and are capable of being carried through the air for short 

 distances, producing- in certain instances disease in other individuals. 

 There is no doubt that in diseases such as smallpox, measles, scarlet 

 fever and other acute exanthematous diseases together with such 

 diseases as plague and diphtheria, that the infectious agents may be 

 carried through the air after having been deposited on clothing and 

 furnishings. However, recent investigations have shown that this 

 method of transferring infection is comparatively rare and that most 

 infections are transmitted by direct contact. 



In the beginning it was supposed that the only way that bacteria 

 could be carried in the air was after having beeil dried on particles of 

 dust and carried by currents of air. This, however, has been shown 

 not always to be the case and we now know that infectious micro- 



