84 



PRINCIPLES OF BACTERIOLOGY 



is practically free from organisms, showing that the air no longer exerts 

 any influence on them. 



Fig. 61 shows a cross-section of the hanging drop shown in Fig. 60. 

 The clumps are evenly distributed throughout the drop, with perhaps 

 some increase in the numbers and compactness of the clumps at the 

 bottom. 



Fig. 62 shows the microscopic appearance of the top of a hanging 

 drop of a bouillon culture to which has been added some blood of a 

 patient suffering from a febrile condition not caused by typhoid infec- 

 tion, but which exerts a slight non-specific influence on the typhoid 

 organisms. It will be seen that the reaction is incomplete and that 

 there are many organisms at the edge of the drop. The air exerts the 

 same influence on the bacilli that it did before the addition of the 

 blood. Note the character of the clumps, generally small and com- 

 pact at the centre, with the bacilli at the edge of the clump, usually 

 attached by one end only. 



FIG. 62 



FIG. 63 



Microscopic field, showing the top of a drop of 

 culture with reaction not due to typhoid. 



Microscopic field, showing a cross-section 

 of Fig. 62. 



Very frequently these clumps have the appearance of being built] 

 up around a piece of detritus present in the clump. All the organisms 

 comprising the clump seem to have retained part, at least, of their 

 motility, those on the edges being particularly motile, so far as their 

 free ends are concerned. 



When motility is very much inhibited these clumps have a peculiar 

 trembling movement, which is like the molecular movement described 

 by Brown. 



Fig. 63 shows a cross-section of the drop represented in Fig. 62.1 

 Note the same character of the clumps in every focal plane: the large 

 number of motile bacilli and the number attracted to the edge of the 

 drop by the air. 



