434 LIFE • OUTLINES OF GENERAL BIOLOGY 



of the invaders is strictly localised, and they dissolve away cellular 

 material at a particular spot till a breakage or lesion results, as may 

 be illustrated by the bursting of a superftciad sore. Sometimes they 

 multiply so quickly that they block a passage, as the bacteria of 

 diphtheria do— and till lately had to be left to do— in the child's 

 windpipe. Sometimes they produce a toxic secretion which has 

 a poisoning effect far and wide throughout the body. In yet other 

 cases there is both a local and a general disturbance. 



But the most important fact is that bacteria produce poisonous 

 secretions which bring about a fatal dlstiubance in the colloidal 

 proteins of the living cells. There is nothing so disturbing to the 

 equilibrium of a cell (or of the blood) as the introduction of a strange 

 protein; it produces what is often called a "protein shock". As we 

 eat a great variety of proteins, it might be thought for a moment 

 that we should be always having protein-shocks, but this is avoided 

 by the breaking up of the proteins into amino-acids in the process 

 of digestion. Hence, taken internally, even snake venom is 

 digested like white of egg, and is no more poisonous. 



Virulent bacteria are rarely, if ever, able to enter an animal 

 through the skin, unless there is some crack or bruise; but a very 

 common mode of infection is when an insect or a tick makes a 

 puncture. In such cases there may be an introduction of a specific 

 parasite, as when the mosquito introduces the malaria Protozoon, 

 or the tsetse-fly the sleeping sickness Protozoon, or the rat-flea 

 the bacillus of the plague. In other cases, where the insect is not 

 known to be the habitual carrier of any specific parasite, there may 

 be an introduction of a casual micro-organism which happens to 

 have found lodgment about the insect's mouth-parts or sting. Thus 

 many people have suffered from blood-poisoning after being stung 

 by the relatively harmless wasp. 



There are some interesting external protections, as in the case of 

 young lampreys, where the mucus secreted on the skin has a strong 

 bactericidal effect. But apart from infection through lesions and 

 punctures, there is not much likelihood of intruders getting in 

 through the outer walls of the body. 



The first great battle-ground is in the food-canal, where some 

 microbes are able to multiply — in spite of the digestive juices — with 

 sinister rapidity. But the conflict becomes still more serious when 

 the intruders get past the ramparts of the body, and even beyond 

 its (still outer) digestive moat. There comes to be fighting in the 

 canals of the circulation, and even within the inmost homes of life; 

 and as the invaders are on the increase during the battle it often 

 goes hard with the organism. Hence the importance of the "internal 

 defences", among whicli a first place must be given to the activity 

 of the phagocytes. 



In all many-celled animals, from sjxjnges to man, with the excep- 



