1154 PHYSIOLOGY 



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 until the animal is able to withstand 

 a dose one hundred thousand times as large a.s that which would have 

 been fatal to it in the first instance. When a condition of immunity 

 has been produced in this way it is found that the blood-serum of the 

 animal has the power of neutralising the toxin. Thus if the blood- 

 serum from a horse which has been treated with large doses of diph- 

 theria toxin be mixed with an equal quantity of the toxin itself, the 

 mixture may be injected into susceptible animals without the produc- 

 tion of any effect. It is possible in this way to get a serum 1 c.c. of 

 which will neutralise many fatal doses of the toxin, and the antitoxic 

 serum may be injected into a susceptible animal and used to confer 

 an artificial immunity on the latter, or may be injected into a diseased 

 animal and used thus as a curative agent. Antitoxin thus plays a 

 great part in modern therapeutics, especially of diphtheria. In the 

 case of tetanus the toxin has a specific affinity for the nervous system 

 and apparently travels up the axis cylinders of the nerves to the central 

 nervous system. By the time that it has arrived at the central 

 nervous system, and the spasms typical of tetanus have broken 

 out, the toxin is already so firmly bound to the reacting tissue that 

 the injection of antitoxin into the blood-stream has little or no 

 effect on the course of the disorder. The use of the tetanus antitoxin is 

 therefore chiefly as a prophylactic agent. 



The question of the manner in which the antitoxin is able to com- 

 bine with and neutralise the toxin is one of considerable practical 

 importance. In this process we have relations presenting marked 

 analogies with the neutralisation of acids by bases. If we define a 

 unit of toxin as that amount which possesses a certain power, i.e. 

 which will kill a guinea-pig in so many days, or will cause the complete 

 haemolysis of 1 c.c. of blood in two and a half hours, we can find the 

 amount of anti-body which is just sufficient to neutralise this effect, 

 and this amount of anti-body can be regarded also as one unit. If 

 instead of one unit of each we take 100 units, the neutralisation is 

 effected in the same way. The process is found, however, to be more 

 complex when we take 100 units of toxin or lysin and attempt to neu- 

 tralise them by the fractional addition of antitoxin. In the case 

 of a strong acid and strong alkali we know that if 100 c.c. of alkali 



