204 BIOLOGICAL CHEMISTRY 



sufficient sulphuric acid to combine with them and render 

 them harmless (p. 150). 



Metchnikoff suggests that chronic toxaemia by bacterial 

 putrefaction may have an influence in hastening the degener- 

 ative changes in the body characteristic of old age.* He 

 further suggests that the activity of putrefactive organisms 

 can be inhibited by weak acids such as lactic acid and that 

 administration of bacteria that can produce lactic acid kills 

 the putrefactive bacteria. f 



The ripening of cheese and other processes due to micro- 

 organisms must not be forgotten. 



In addition to the micro-organisms which grow outside the 

 body there are those which grow inside the living organism 

 and produce a series of chemical changes. The mere presence 

 of micro-organisms is usually harmless, but they produce their 

 bad effects by poisons or toxins. The toxins may produce a 

 rise of temperature and other symptoms depending on the 

 nature of the toxin produced. The toxin is usually peculiar 

 to the species of micro-organism that produces it and the 

 specific action is frequently very marked. In some cases 

 special groups of cells are affected before others ; for instance, 

 the toxin of tetanus bacteria tends to attack the cells of the 

 facial nerve in the medulla, and the toxin of diphtheria bacteria 

 frequently acts upon the nerves to the soft palate. 



The defence of the body against bacterial invaders consists 

 of two methods, the first a biological process and the second 

 a series of chemical reactions. The biological method must be 

 considered here, as there are certain chemical points connected 

 with it. Certain of the white corpuscles of the blood can 

 undergo what are termed amoeboid movements, that is flowing 

 movements whereby they can surround and engulf particles. 

 The presence of bacteria in the tissues causes the white 

 corpuscles to escape from the blood vessels and move towards 

 the invaders. The stimulus to these movements is the diffu- 

 sion of substances from the bacteria and is thus similar to 

 taste or smell : it is called chemotaxis. 



If the corpuscles engulf the bacteria we speak of it as 

 phagocytosis, and this is an important method of destroying 

 bacteria. The phagocytic corpuscles have been called the 

 policemen of the body. J If, on the other hand, the corpuscles 



* E. Metchnikoff, The Nature of Man, translated by P. C. Mitchell. 

 Heinemann, 1903. 



| A. Rovighi, Zeit. physioL Chem., 1892, vol. 16, p. 20. 



I E. Metchnikoff, L'lmmuniU dans les Maladies Infectieuses. 

 Paris, 1901. 



