DISEASE-RESISTING FUNCTIONS OF THE BLOOD 307 



tissues of the Body form, however, excellent culture media; or- 

 ganisms that do establish themselves flourish mightily, at least for a 

 time. It follows, therefore, that ordinarily organisms are forcibly 

 prevented from establishing themselves. This prevention of infec- 

 tion is in part a function of the tissue generally and in part a func- 

 tion of the blood. It must be sharply differentiated from an 

 additional disease-resisting function possessed also by the blood, 

 which is the overcoming of infection after it is once established. In 

 the absence of this latter function every infection would result 

 fatally. 



The Infection-Resisting Mechanism. Although, as stated 

 above, the bodily tissues form excellent culture media they do not 

 all yield readily to the attacks of the invading micro-organisms. 

 Some tissues are more susceptible than others, and some kinds of 

 organisms attack certain tissues more readily than they do others. 

 The curious fact has recently been demonstrated that the organism 

 of " blood-poisoning " or septicemia, which often attacks nearly all 

 the tissues of the Body, shows a decided preference for the par- 

 ticular tissue in which it formerly grew. Thus if from an animal 

 killed by the infection some of the organisms found in the kidney 

 be injected into the veins of a second animal, the kidneys of the 

 latter will be first attacked. If the organisms came from the liver 

 they will strike first at the liver. 



The tissues of some people are in general more resistant than 

 those of others. It is believed that this resistance is to a certain 

 degree inherited. At any rate the experience of peoples exposed for 

 the first time to particular infections suggests this. Whenever in 

 the history of the world races have been brought into contact with 

 new diseases they have suffered severely therefrom, although in 

 many cases the diseases which wrought the havoc were lightly es- 

 teemed by races that had been accustomed to them for generations. 



In addition to this general mode of resisting infection, which 

 we may call tissue resistance, there are two sorts of structures in the 

 blood specially devoted to the destruction of invading micro- 

 organisms. They work independently but in co-operation. The 

 first of these are the phagocytes previously mentioned (p. 302) 

 which engulf^and thus dispose of the invading foreign bodies. The 

 second sort is not made up of formed elements like the phagocytes, 

 but is in solution in the plasma. It attacks and destroys bacteria 



