29 BACTERIOLOGY 



in the tissues and fluids of the human body, as well as ex- 

 ternal to it. It still remains for us to give a brief account of 

 those methods which are employed to ascertain the special 

 significance of the different micro-organisms for the human 

 body, that is to say, to recognise by means of experiment 

 their pathogenic powers. 



Amongst micro-organisms a distinction is drawn, as 

 we have learnt, between those which exercise a specific 

 injurious influence upon the bodies of men and animals, and 

 those which do not possess this property, although they may 

 perhaps occasion disturbances of various kinds by their 

 numbers ; the former being known as parasites, the latter 

 as saprophytes. 



In order, then, to investigate micro-organisms with 

 reference to their power of causing disease, experiments 

 must be made by transmitting them to animals, for which 

 purpose monkeys, dogs, cats, hedgehogs, rabbits, guinea- 

 pigs, white mice, rats, marmots, poultry, pigeons, or even 

 frogs (kept at abnormally high temperatures) are used. 

 [To prove that a particular micro-organism is the specific 

 cause of a given disease it should be shown l 



1st. That its presence can be detected with the micro- 

 scope in all cases of that disease. 



2nd. That it is never found in any other disease. 



3rd. That when isolated and cultivated through many 

 generations a culture inoculated on a susceptible animal 

 invariably produces a disease identical with that in the 

 animal from which the virus was taken, and 



4th. That the same bacteria are found to be present in 

 the animal so inoculated.] 



Transmission can easily be effected on the cutaneous 

 surface, or on the mucous membranes of readily accessible 



1 [See on this subject Giinther's Einfilhr. in das Stud, der BakterioL, 

 pp. 139 et seq., 2nd ed.j Tn. 



