6 MANUAL OF BACTERIOLOGY. 



by certain bacilli that invaded the body just before or just after 

 death (bacillus aerogenes capsulatus). 



Woodhead tells us that some savages are in the habit of smear- 

 ing the soil of certain localities upon their arrows for an arrow- 

 poison, which is intelligible in the light of the fact that soil often 

 contains the bacilli of tetanus (lockjaw). 



The comparatively small number of species of bacteria that 

 cause disease are the ones that interest us most, and are those 

 which have been most carefully studied. The necessity that 

 falls upon bacteria, in common with other fungi, to derive their 

 food from organic matter makes it easy to understand that they 

 should frequently exist as parasites upon living animals and 

 plants. Pear-blight and some other diseases of plants are 

 caused by bacteria. We find that frogs, birds, cattle and a 

 great number of animals besides men suffer from diseases pro- 

 duced by bacteria. 



When bacteria are placed upon slips of glass they may be 

 studied with the microscope while ahve. Some of them when 

 living are motionless; others wriggle vigorously. Some dart 

 about like minnows in a stream, or they make their way slowly 

 across the field of the microscope like a boat that is being 

 sculled from the stem. By proper methods it can be shown 

 that the movements are effected through one or more fine, hair- 

 like processes called fiagella. 



Often it is expedient to study bacteria after drying them 

 on slips of glass, when they may be made more conspicuous 

 by giving them an artificial color (staining). Some of the 

 substances of which they are composed readily absorb certain 

 dyes. For this purpose the anihne dyes are used, and their 

 employment has been one of the important factors in making 

 progress in bacteriology possible. 



With the microscope alone it is not usually practicable to 

 distinguish accurately between various kinds of bacteria. 

 Micrococci, for instance, which are, in reahty, extremely 



