154 TEXT-BOOK OF BACTERIOLOGY. 



question. In the same way, when a particular micro-organism 

 affects an animal, we must not expect to see precisely the same 

 phenomena produced which we were accustomed to observe in 

 human beings. Occasionally chance comes to our aid and brings 

 about an unintentional transmission to a human being, or some 

 particularly zealous investigator goes to the length of experiment- 

 ing upon himself. As a rule, however, we must direct our attention 

 chiefly to the animals more nearly related to mankind, and the 

 example of relapsing fever, which can be transmitted to human 

 beings and monkeys alone, would seem to indicate that we are 

 on the right road. 



Monkeys, however, are expensive animals and therefore not numer- 

 ous in our laboratories. As a rule, we keep mice, Guinea-pigs, rab- 

 bits, and occasionally a few varieties of poultry (pigeons and fowls). 

 These are the ordinary materials from which we seldom depart, 

 and it is astonishing that so much success has, nevertheless, been 

 obtained with these restricted means. The reason is probably that 

 the animals just named are, generally speaking, easily susceptible 

 to infection, whereas the dog, which was formerly almost exclu- 

 sively employed, proved far less satisfactory, being insusceptible 

 to the influence of most organized poisons. 



Another cause of failure in numerous attempts at transmission 

 has been the defective manner of conducting them. 



It is by no means immaterial in what manner we apply the in- 

 oculating matter: we must here, too, endeavor to imitate the 

 operations of nature so far as they are known to us. There are 

 three ways in which micro-organisms usually penetrate into our 

 bodies : 



First, from the surface of the skin, generally after it has been 

 injured in some way i.e., through a wound, from whence the 

 bacteria find their way into the blood or juices. It does not, in- 

 deed, always require such a special door of entry. The investiga- 

 tions of Garre, Schimmelbusch, Roth, Braunschweig, and others, 

 have proved that the uninjured skin or mucous membrane is pene- 

 trable to infectious germs, and therefore it does not offer the un- 

 conditional protection often attributed to it. 



Second, the digestive canal, into which the bacteria pass along 

 with the food. Many, it is true, cannot pass through the stomach 

 in their usual form, being destroyed by the action of its acid con- 

 tents. Other kinds are less sensitive, and when spores are present, 

 or when disease has altered the character of the digestive fluid and 

 weakened its bacteria-killing power, there is no further obstacle 

 to the passage of the parasites. 



