IMMUNITY 195 



protozoa (unicellular organisms from the animal world) can take up a 

 sort of parasitic life within other living forms. Whether a given sub- 

 stance injures chemically, or whether a living organism is to find its 

 way into another and injure it, depends upon whether or not the foreign 

 substance or organism can penetrate through the outer skin surface of 

 the body or through the surface of the gastro-intestinal-tract. It is only 

 when such foreign objects are able to get within the body proper (that 

 is, within the space between body surface and intestinal surface) that 

 injury results. 



Probably most of such injurious substances are taken in through 

 the mouth and later find their way through the more or less delicate 

 lining of the gastro-intestinal-tract. It is of value to note that the 

 seventy of a burn, in the area affected, depends upon the length of time 

 the particular area remains in contact with either fire or acid. This 

 means that a deep or severe burn in a localized area may cause death, 

 while a less severe burn spread over a greater area might not. 



Now, if a liquid, which can neutralize or wash away the given sub- 

 stance, could be thrown upon the acid at the time it is spilled, such acid 

 would be washed away immediately and little if any harm would be 

 done. So, too, in the gastro-intestinal-tract, an injurious substance that 

 may find its way therein, may be neutralized or washed away if a suf- 

 ficient quantity of neutralizing fluid is secreted or passed through the 

 intestinal tract. 



Therefore, two things must be kept in mind when discussing a sub- 

 ject of this kind: the strength or power of the injuring agent, and the 

 length of time the injuring agent is in contact with a susceptible surface. 

 In fact one may add a third factor, for there is a possibility of a foreign 

 substance being taken into the system which may so affect the regen- 

 erative abilities of the host, as not only to prevent healing of a wound, 

 but which will actually continue to irritate and injure more than the 

 original injuring agent. 



Once the injuring agent has entered the body the question arises 

 as to the method by which it injures -the host. It must be remembered 

 that living organisms, whether they be bacteria or protozoa, are in turn 

 subject to the same laws that govern the life of the host itself. Some 

 of the larger parasites, such as tapeworms, really remain within the in- 

 testinal tract and use the food of the host before the host himself derives 

 the benefit of what he has eaten. There are also parasitic protozoans, 

 such as the malarial parasite, which, once it has entered the blood 

 stream, actually eats out the center of the blood cells. Then there are 

 those which use some part of either the blood or other tissues of the 

 body for food and in this way injure the host; or again, there are those 

 which use but a very small quantity of the host's food and are conse- 

 quently not particularly injurious to the host on that account, but the 

 various excreta ejected by these parasites may prove injurious either as 



