THE CHEMICAL MECHANISMS OF DEFENCE 1031 



bacteria is attended with poisonous effects. The bacteria may be thus 

 classified according as they produce extracellular or intracellular toxins. 

 We may deal first with the manner in which the body reacts to the toxins 

 excreted by the first class. If a culture of diphtheria or tetanus bacilli be 

 filtered, the clear filtrate free from bacilli is found to exercise as poisonous 

 results as if the culture itself of the living bacilli had been employed. The 

 toxins contained in these fluids are extremely potent. Thus five-millionths 

 of a gramme of tetanus toxin is a fatal dose for a mouse, and *00023 grm. 

 would kill a man. These weights apply to the mixture obtained by the 

 evaporation of the solution of toxin, so that the pure toxin must be even 

 more powerful than is represented in these figures. We have at present no 

 means of preparing a toxin in a pure condition, nor do we know to what class 

 of compounds it should be assigned. The toxin is an unstable body and is 

 destroyed by heating to 65 C. Similar toxins are widely distributed 

 throughout the vegetable and animal kingdoms. Thus they form the 

 active constituent of snake venom and of the poison of scorpions and 

 spiders. They also occur in the seeds of castor oil and of jequirity, the 

 toxins of which seem to be of protein character and are known as ricin and 

 abrin. There is a great variability in the reaction of different animals to 

 these toxins. Thus to the poison of tetanus the rabbit is weight for weight 

 two thousand, times and the hen twenty thousand times more resistant 

 than the guinea-pig. As in the case of infection by bacteria themselves, 

 a certain incubation time is necessary after the introduction of the toxin 

 before its effects are displayed. There is a striking difference in this respect 

 between the action of these complex bodies and the action of drugs, such as 

 strychnine or morphine. Thus by increasing the dose of strychnine it is 

 possible to kill an animal within half a minute. The period of survival 

 after the injection of a dose of toxin cannot be reduced beyond a certain 

 limit, however much toxin be injected. Thus a lethal dose of diphtheria 

 toxin kills a guinea-pig in fifteen hours. If ninety thousand such doses be 

 injected into a guinea-pig it is not possible to reduce the time of survival 

 below twelve hours. Another characteristic of these toxins is the specificity 

 of their action. One kind of toxin may act chiefly on the central nervous 

 system, another on the peripheral nerves, another on the red blood-corpuscles. 

 In this respect of course they resemble ordinary drugs. Associated with, 

 however, and apparently a necessary condition of, this specific action is the 

 actual combination which occurs between the toxin and the organ on which it 

 exerts its effect. Thus tetanus toxin has a specific affinity for the central 

 nervous system, and may be removed from a solution by shaking the latter 

 up with an emulsion of brain. In spite of the excessively fatal character of 

 these toxins it is possible to render an animal immune to their action. 

 If a dose of diphtheria or tetanus toxin which is smaller than the fatal dose 

 be injected into an animal, the latter may show signs of injury from which 

 it recovers. When recovery is complete it is found that three or four times 

 the fatal dose may be injected without producing any evil effects, and this 

 process of injection of toxin may be repeated in continually increasing doses 



