246 POPULAR SCIENCE MONTHLY 



It is an interesting subject of speculation as to what the result will 

 be when cattle in general, and possibly, man later, shall have been 

 immunized to tuberculosis. Will the race of tubercle bacilli disappear 

 in large measure from the world? This would indeed be a beneficent 

 result. But Dr. Smith has pointed out in a recently delivered address 

 that doubtless host and parasite eventually come to hold a kind of 

 equilibrium to each other, and hence an increased degree of resistance 

 in the former might tend to bring about that selection among the para- 

 sites through which races of greatly augmented power for invasion 

 would be produced. If this were true, and he even suggests that the 

 natural process of weeding out the weaker among the human race tends 

 to this result, the parasite would try to keep up with the host as his 

 resistance increased until a point was reached beyond which further 

 enhancement of power was impossible. Would the higher animal or 

 the lower vegetable organism finally claim the victory? We need per- 

 haps at this moment not to relax our efforts to achieve a practical 

 immunity for man as well as for animals because of this future dan- 

 ger. I am not aware that the smallpox germ has increased measurably 

 in virulence since vaccination became general, but I would also add 

 that a century is a small period of time in the life history of any 

 living organism. 



Before closing this address I should like to refer briefly to the new 

 interest which has been excited in the use of tuberculin in the treat- 

 ment of human tuberculosis by reason of the application to the study 

 of tuberculosis of a method introduced by E. A. Wright, of London, 

 whereby it is held that the exact effect of the tuberculin injection can 

 be measured and controlled. The method consists in the determination 

 of the capacity of the blood leucocytes to take up tubercle bacilli when 

 the blood and the bacilli are brought together outside the body in a test 

 tube. Wright and his pupils have worked out the normal power of 

 the blood to cause the englobing of the bacilli; and they have noted a 

 diminution of this capacity in the blood of many persons suffering 

 from tuberculosis. They speak of this englobing capacity of the blood 

 as ' opsonic index,' from the word meaning to prepare — to cater for ; 

 since the bacilli must first be prepared by substances in the blood serum 

 before they can be ingested by leucocytes. The injection of tuberculin, 

 when cautiously done, tends to bring about a rise in the tuberculous, 

 of the ' opsonic index/ which Wright believes is a measure of the good 

 done, as an increase in immunizing substances in the blood is the cause 

 of the rise. He also discovered that time is required for the occurrence 

 of the rise and that the immediate result of the injection is a fall of 

 the index — so-called negative phase. This latter must be permitted to 

 pass away and be succeeded by the positive phase before another injec- 

 tion is given. Gradually the ' opsonic index ' is driven up in the cases 

 that are favorable to the treatment. 



