154 BACTERIA 



complex ones. In this circulation the animals play 

 a comparatively small part, for they merely convert 

 the plant proteins into the proteins of their own bodies. 



Besides the circulation of nitrogen there is also a 

 circulation of carbon, which appears in the form of 

 carbon dioxide, not only as the result of the respiration 

 of plants and animals, but also during decomposition 

 of the complex carbon compounds of animal and plant 

 bodies, notably the carbohydrates found in the cellulose 

 of plants, but also the fats and proteins. Distinct bac- 

 teria carry out the disintegration of the cellulose of 

 which humus is largely composed, and this process 

 ultimately results in the production of carbon dioxide 

 and water (as well as hydrogen and marsh gas). The 

 carbon dioxide is then, as we know, used by the green 

 plant to build up its body. 



Pathogenic (Disease-producing) Bacteria. A small 

 proportion of the total number of species are partly 

 or wholly parasitic on plants and animals in which 

 they cause disease. Partial parasites (such as Bacillus 

 tetani) live naturally in soil containing organic matter 

 and only occasionally gain entrance to the animal 

 body. Complete parasites (such as the spirochaete of 

 syphilis, or the coccus of cerebro-spinal fever) have no 

 life outside the body, and their existence depends on their 

 being passed on directly from one person to another. 



Bacteria often invade living animal bodies by way 

 of wounds, and may be a great danger to life, for in- 

 stance on battlefields, owing to the poisons or toxins 

 which they produce. The bacteria are carried into 

 the wound either by the instrument causing the wound, 

 or from the clothes, or by dirt afterwards getting on 

 to the wounded surface. The most important are the 

 species of Streptococcus and Staphylococcus ; and these 



