PARASITIC DISEASES OF THE HORSE. 433 



nitrogen, phosphorus, sulphur, calcium, magnesium, and potas- 

 sium. The presence of water is essential to the development of 

 all fungi, as a medium for conveying oxygen and food sub- 

 stances into the cell. Each organism flourishes best at its special 

 temperature. It is held that bacteria do not reproduce at 5° C, 

 and many require a much higher point; but they do not neces- 

 sarily die at lower temperature. Some are not killed by the 

 greatest cold, and it is uncertain that any die from extreme cold. 

 Some bacteria are frozen in fluids at minus 110° C. without in- 

 jury- 



These organisms are found in putrid wounds, in which they 

 fairly swarm. They may enter from the exterior, developing 

 only under special circumstances; or they may be spontaneously 

 generated in the body under special circumstances, from ele- 

 ments of the tisues. Earth, air, or water may be the habitat of 

 germs external to the liody. They exist in earth in the neigh- 

 borhood of putrefaction. Koch found that all organisms are 

 absent at a depth of one meter in soil not recently disturbed in 

 winter, and not formed largely of decomposing material, and into 

 which no usual leakage of water occurs. 



Dust contains much organic matter, as is easily shown by com- 

 bustion, and cultivation proves that some of this is living. The 

 air is supplied with organisms swept by currents from objects 

 over which they pass. The dust left as the final result of putre- 

 faction is a fertile source of contamination. Perfectly still air 

 becomes pure by subsidence of its germs. All water, except such 

 as comes from a gTeat depth, as in Artesian wells, contains or- 

 ganisms. Rain water sweeps the air and infects the soil with the 

 germs which it carries down. All surface water is infected from 

 the ground through which it soaks, and too often shallow wells 

 are contaminated by sewerage. Tviver water is ex]iosod to all 

 possible sources of pollution. 



These organisms exist in large numbers on the external skin, 

 and internally on the bronchial and alimentary surfaces which 

 are in contact with air. They are carried into the alimentary 

 28 



