THE ENVIRONMENT OF BACTERIA. 65 



the dreaded tubercle bacillus which can only leave the 

 mouth, with the sputum, or in the small particles of liquid 

 which may be scattered about in a fit of violent coughing-. 



The surface layers of the soil are exceedingly rich in 

 bacteria. These organisms decrease rapidly in numbers 

 with the depth, and eventually disappear entirely. The 

 soil at, or below, a depth of 8 or 10 feet can be regarded as 

 practically free from bacteria. This is due, in the first 

 place, to the fact that the earth acts as a filter retaining 

 most of the organisms at, or near, the surface. It is also due, 

 in part, to the unfavorable conditions, such as low tem- 

 perature and insufficient supply of oxygen. The rain, or 

 melting snow, percolates through the soil and eventually be- 

 comes wholly free from the suspended organisms. It is clear, 

 therefore, that the water which comes from the deeper 

 layers of the earth is free from bacteria. The water of a 

 spring, or of a tubular, or artesian well may contain some 

 organisms, but these are due to accidental contamination at, 

 or near, the surface. 



The higher animal inhales every day an enormous num- 

 ber of bacteria. Moreover, these organisms are constantly 

 being introduced into the alimentary canal with the food. 

 The intestinal contents, as a result, are extremely rich in 

 bacteria. The surface of the body may also harbor a large 

 number of these organisms. In spite of the fact that the body 

 is thus besieged on all sides by countless bacteria, it is never- 

 theless free from them. In a healthy individual the blood 

 -and lymph, the various internal organs and tissues are 

 wholly free from bacteria. It follows, therefore, that the 

 secretions, such as the urine and milk, of a normal individual 

 are perfectly germ free. 



