•224 HEALTH AND DISEASE 



class of fungi. In the horse there are only three or four diseases which 

 have been shown to depend upon a disease-producing microscopic organism, 

 which in each case is distinguished by certain peculiarities of form and 

 modes of growth. The diseases are anthrax, glanders, and tuberculosis. 

 Strangles is the consequence of the introduction into the system of a 

 pus (matter) producing organism belonging to the streptococci. On the 

 basis of the fact that some contagious maladies depend for their existence 

 on a living organism, the pure cultivation of which outside the body will 

 produce the disease when inoculated into a susceptible subject, it has been 

 assumed that all contagious diseases depend on the presence of similar 

 living beings. This, however, remains to be proved with regard to a con- 

 siderable proportion of contagious maladies. Small-pox, scarlatina, rabies, 

 for example, and vaccinia, have not up to the present time furnished 

 characteristic microbes, although the search for them has been pursued for 

 a long time past, and is still being carried on with the utmost diligence by 

 experts in different parts of the world. 



BACTERIA 



Although disease-producing micro-organisms take their place naturally 

 among the exciting causes of disease, their life -history has attracted so 

 much attention during the last twenty years, and has been so exhaustively 

 studied during the last ten years, that it is imperative that they should 

 receive special consideration. The discovery of the microscope, which, 

 according to Professor Edgar Crookshank, was an event of two and a 

 half centuries ago, was as a matter of course followed by the detection of 

 organisms in animal fluids and elsewhere, the existence of which previously 

 could only be suspected. Shortly after the microscope was first brought 

 into use, it was found that small living things were abundantly present in 

 all decomposing substances, and Kircher believed that similar organisms 

 could be found in various diseases. His researches were directed to the 

 discovery of such organisms; but the modern microscopist would conclude 

 without hesitation that Kircher's chances of success were extremely remote 

 with the very primitive form of optical appliances which were then at Ins 

 command. The knowledge of the forms and functions of bacteria advanced 

 along with the progress in the development of the microscope, and a 

 considerable step was taken when Anthony Van Leeuwenhoek devoted his 

 attention to the construction of lenses, and made such improvements in the 

 microscope as earned for him the title of the father of microscopy. In 

 1675 he described, in a series of letters to the Royal Society, numerous 

 minute organisms in rain water, well water, infusions of pepper and hay, 



