102 EXPERIMENTS UPON ANIMALS. 



mixed infection or whether a pure culture of a single microorganism 

 is obtained from the blood. In the same way cultures may be made 

 from material obtained from the liver or spleen, and it may happen 

 that one or both of these organs contain bacteria when none are 

 found in the blood. Before passing the platinum needle or collect- 

 ing tube into the organ, the surface, which has been more or less ex- 

 posed to contamination, should be sterilized by applying to it a hot 

 spatula ; then at the moment of lifting the spatula the sterilized 

 needle is introduced into the interior of the organ, and the blood and 

 crushed tissue adhering to it at once carried over to the culture me- 

 dium. Or blood obtained with proper precautions from a vein, a 

 cavity of the heart, or the interior of the spleen or liver, may be 

 used to inoculate another animal. 



Animals are also sometimes inoculated by excoriating the cutis 

 as in vaccination. They may also, in rare cases, be infected by in- 

 troducing cultures into the stomach, either mixed with the food in- 

 gested or by injection through a tube. Infection by inhalation is 

 accomplished by causing the animal to respire an atmosphere, in a 

 properly enclosed space, in which the pathogenic organism is sus- 

 pended, by the use of a spray apparatus for liquid cultures, or 

 some form of powder blower for powders containing the bacteria in 

 a desiccated condition. 



One method of obtaining a pure culture of pathogenic bacteria 

 consists in the inoculation of susceptible animals with material con- 

 taining a pathogenic species in association with others which are not. 

 When the blood is invaded by the pathogenic species and the animal 

 dies from an acute septicaemia, we may usually obtain a pure cul- 

 ture by inoculating a suitable culture medium with a minute drop of 

 blood taken from a vein or from one of the cavities of the heart. 

 Sometimes, however, a mixed infection occurs and some other mi- 

 croorganism is associated in the blood with that one which was the 

 immediate cause of the death of the animal. 



