32 BACTERIA AND THEIR PLACE IN NATURE 



brane. Normally, the infant enters the world free from bacteria, 

 but they soon begin to settle on the skin; they penetrate the nose 

 and mouth; the first respiratory movements and cries carry them 

 into the respiratory passages; and between the tenth and seven- 

 teenth hour they have reached the intestines. 



Ordinarily, the deeper respiratory passages contain but few 

 bacteria, but it has been proved that even the tubercle bacillus can 

 penetrate with the inspired air to the bottom of the pulmonary 

 alveoli. 



On account of its acidity, yeasts and molds nourish better in the 

 stomach than do bacteria. However, at least thirty species of 

 bacteria (occurring in the stomach) have been described, many of 

 which have attracted special attention on account of the belief that 

 their presence may favor other more injurious species. 



The intestines, on account of their alkaline reaction and the partly 

 digested condition of their contents, are a great reservoir of bacterial 

 activity. Metchnikoff and others have given an immense amount 

 of work to a consideration of their function within the body and the 

 probable result in their absence. The only conclusion which is 

 possible at present is that, living as we are in a world filled with micro- 

 organisms, life without them is impossible. All that can be done is 

 to make conditions such that the injurious species are suppressed 

 and the beneficial ones favored. Out of this has grown sour-milk 

 therapy. 



The normal tissues of plants and the blood and tissues of animals 

 are free from bacteria. They are rarely found on certain healthy 

 mucous membranes, such as those of the kidney, bladder, and 

 lungs. Occasionally they pass through the skin or .the mucous 

 membrane of the digestive tract after which they may be found for 

 a short time in the blood. This is especially the case during the 

 height of digestion and it probably accounts for the large number 

 of leukocytes which swarm in the intestinal mucosa and which have 

 been thought to be in some way associated with the process of fat 

 absorption. 



In certain diseased conditions the blood and many of the tissues 

 of the human body are found to contain numerous bacteria. Soon 

 after death even the saprophytes rapidly invade and decompose the 

 body tissues. 



R61e of Bacteria in Nature. Bacteria play a wonderful role in the 

 many transformations going on in this world. It is difficult to con- 

 ceive of life without them and their help. For the beneficial ones 

 we turn first to the soil, for from this either directly or indirectly 

 man largely draws his food, clothing, and other necessities of life. 

 The soil is not, as many think, a dead, inert mass, but it is teeming 

 with life! Both microscopic plants and animals inhabit it by the 

 millions. These have been at work within it long before man began 



