BIOLOGY OF BACTERIA. 63 



3. Actual penetration of the vessel-wall by the growth 

 of the microorganism. 



4. Entrance into the vessels via the lymphatics, either 

 passively or in leucocytes. 



Seeing that the channels by which bacteria can enter 

 the body are so numerous, and that there 'is scarce a 

 moment when some part of us is not in contact with 

 them, how is it that we are not constantly subject to 

 disease ? The consideration of this question, together 

 with the closely related questions why we should be 

 subject to certain diseases only, and to these diseases 

 at certain times only, must be reserved for another chap- 

 ter, in which the subjects Immunity and Susceptibility can 

 be taken up at length. Before passing on to it, however, 

 some attention must be paid to the subject of the 



Elimination of Bacteria from the Body. There is every 

 reason to think that non-pathogenic bacteria entering the 

 body ordinarily, or being experimentally injected into it, 

 follow the same course as inert, non-vital particles; con- 

 cerning which, the experiments of Siebel have shown 

 that they accumulate in the finest capillaries, especially 

 in the lung, liver, spleen, and bone-marrow, and are 

 slowly transferred to the surrounding tissues, either to be 

 collected in the connective-tissues, carried to the lym- 

 phatic nodes, or to be subsequently excreted with the 

 bile, succus entericus, etc. , or to be discharged from the 

 surface of the mucous membranes, pulmonary alveoli, 

 tonsils, etc. They also escape from suppurating wounds 

 to which they may be carried by leucocytes. They are 

 not excreted by the kidneys. 



The experiments of Wyssokowitsch are in accord with 

 the results of Siebel' s work, and show that the kidney 

 rarely eliminates bacteria. Cavazzani found that the 

 kidney had the power to retain bacteria in the blood, 

 unless the epithelium was injured. 



The principal avenues of escape for the bacteria are, 

 therefore, for the non-pathogenic forms, the mucous mem- 

 branes, the bile, and the sweat. For the pathogenic 



