PRACTICAL APPLICATIONS 379 



in the circulating blood-plasma, and that the mechanism is of 

 comparatively little value appears from the fact that rupture of a 

 caseous gland into a vein is so often (as far as we know, always) 

 followed by general tuberculosis. It is possible that living and 

 virulent bacilli may be taken up by the polynuclear leucocytes, 

 carried to distant organs, such as the lymph glands or spleen, and 

 there continue to grow, producing anatomical tubercles. 



As regards the nature of tuberculo-opsonin, the normal opsonin 

 is completely thermolabile, and the rise in the index due to 

 injections or the natural disease is mainly of that nature also. 

 Occasionally the presence of thermostable opsonin is demonstrable, 

 but it never becomes abundant. 



We know but little concerning any other antibody or defensive 

 mechanism. No bacteriolytic or bactericidal action is demon- 

 strable. This may possibly be due to the technical difficulties 

 incidental to investigations of this nature on a bacillus which 

 owes its staining properties to the presence of fats (which we 

 cannot expect a serum to dissolve), and which grows so slowly. 

 Wassermann and others have shown that antibodies occur in 

 patients immunized with tuberculin, but we do not know their 

 nature. Nothing really definite is known concerning an antitoxin. 

 An agglutinin is often formed, but it may be absent throughout 

 the whole course of the disease, so that we cannot regard it as of 

 importance. 



Diagnosis. Where possible this should be made by the recogni- 

 tion of the bacillus, an achievement which modern methods 

 have made possible in many cases in which it would formerly 

 have been regarded as out of the question. Into this and into 

 questions of cytology, etc., we need not enter. The main methods 

 to be considered are (i) the tuberculin reaction, and (2) the 

 opsonic index. 



i. The Tuberculin Reaction. The old tuberculin isused,and is best 

 bought ready prepared, as issued with the German Government 

 stamp. It may be standardized, but this process is uncertain, 

 animals differing markedly in susceptibility. Several methods 

 have been introduced, but are not in general use. Behring's 

 method is to determine the lethal dose for guinea-pigs on sub- 

 cutaneous injection, whilst Von Lingelsheim injects directly into 

 the brain, the susceptibility of which is much greater. As a rule, 

 a normal guinea-pig will stand a dose of i c.c. with impunity. It 

 should be diluted before use with sterile normal saline solution 



