INFECTION- BY THE DIGESTIVE CANAL t}3 



cavities, an experiment made in the latter way being often, 

 indeed, attended by better results than follow transmission 

 into small wounds of the skin ; but care must be taken that it 

 does not become possible for the animals to remove the micro- 

 organisms which have been introduced. Whether infection 

 can take place through the epithelial structures of the skin, 

 if unbroken, has not yet been finally decided. 



Infection by the air passages. — Entrance can readily 

 be effected through the respiratory tract ; indeed, infection 

 seems to be able to gain admission with particular ease by 

 the internal surface of the lungs, especially sore ; the degree 

 of moisture all over the surface assists by fixing the micro- 

 organisms and enabling them to develop. For artificial 

 infection by the respiratory passages a spray ajyparatus is 

 used, by means of which the micro-organisms, suspended 

 in bouillon, reach their destination in the form of a fine 

 shower ; but it is not easy to prevent the simultaneous 

 occurrence of a second infection, since during the process the 

 infectious matter may reach the intestinal canal by being 

 swallowed, or may be deposited on the skin. To render this 

 less easy of occurrence the excessively fine mist must be con- 

 ducted by means of a tube into a closed chest in which the 

 animal to be experimented on has been placed, so that it 

 can thus freely breathe in the micro-organisms suspended 

 in the air of the interior space. 



Infection by the digestive canal. — Infection is communi- ' 

 cated through the intestinal tract either in the food or 

 directly by means of an oesophageal bougie, or the micro- 

 organisms may be introduced by establishing a gastric or 

 intestinal fistula. The best mode is, however, to hollow 

 out pieces of potato, fill them with the bacterial culture, 

 and push them so far back into the animal's pharynx that 

 they must be swallowed. Fluid infecting material is ad- 

 ministered to animals by means of oesophageal bougies 



