THE DEFENSIVE MECHANISMS OF THE BLOOD. 149 



the other hand such as syphilis, malaria, sleeping sickness and 

 those caused by amoeba in the mouth and alimentary tract find 

 relatively little resistance offered to the ingrowth in the body, and 

 their destruction therefore must be for the most part brought 

 about by drugs. 



The Process of Inflammation, which in a general way is 

 known by the common symptoms of fever, pain, swelling and 

 redness, is a sign of an increased activity on the part of the tis- 

 sues in an effort to destroy some foreign body which is poisonous 

 to the cells. Microscopical examination of a section of inflamed 

 tissue will show that the blood vessels are dilated, and that the 

 tissue spaces are infiltrated with leucocytes. It suggests that the 

 blood elements must have a very important part in the process. 

 The study of this function of the body is one of the most inter- 

 esting chapters of physiological science, and includes the ques- 

 tions of immunity from disease and the cure of infectious pro- 

 cesses. 



Many pathogenic organisms can be cultivated on artificial 

 media and the products of their metabolism can then be studied. 

 It has been found that they may be divided into two groups ; the 

 one group producing the soluble poisons, or true toxins, which 

 are excreted from the cell ; and the other group producing toxic 

 substances, the endo-toxins, which are not excreted from the cell. 

 We will first take up the manner in which the body deals with 

 the toxins. 



Toxins. If a culture of diphtheria or tetanus bacilli be fil- 

 tered through a porcelain filter, the bodies of the bacilli are re- 

 moved and the filtrate contains the soluble toxic principles which 

 the bacilli have produced and excreted into the nutrient fluid. 

 Injections of a small amount of this filtrate into an animal will 

 produce the same symptoms as are produced when a pure culture 

 of the bacilli is injected. Each bacillus produces a specific kind 

 of toxin. Diphtheria toxin acts primarily on the vascular sys- 

 tem; tetanus toxin, on the central nervous system. The chemical 

 nature of the toxin molecule is unknown, since it has been impos- 

 sible to separate it in pure form. It is probably closely related 

 to the protein molecule, and on the other hand resembles the 



