THE CHEMICAL MECHANISMS OF DEFENCE 1153 



into animals. Other bacteria do not form such extracellular toxins, 

 but in their case it is found that if the bodies of the bacilli be broken up 

 the injection of the contents of the bacteria is attended with poisonous 

 effects. The bacteria may be thus classified according as they pro- 

 duce 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 con- 

 stituent 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 two 

 thousand times and the hen twenty thousand times more resistant 

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

 selves, 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 con- 

 dition 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, 



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