PRESIDENT S ADDRESS SECTION L. 261 



serum of patients infected with typhoid, glanders, contagious abor- 

 tion, and many more diseases, or, as a result of the injection of 

 the dead bacilli of these diseases; the presence of "precipitins" 

 as a result of inoculation with foreign sera, milk, egg albiimen, and 

 other proteins; the development of the power of " bacteriolysis " as 

 a result of infection with living, or of inoculation with dead, bac- 

 teria ; and similarly the increased power of phagocytosis because of 

 the formation of "opsonins" by reason of the antigenic action of 

 infecting organisms; the production of " htemolysins " and " cyto- 

 lysins " following the injection of foreign red corpuscles or other 

 cells ; all these can be put in evidence >u vivo or in vitro as show- 

 ing the immen'^ely increased defensive power of the body against 

 the antigen m question. Often, in fact generally, more than one 

 antibody results from the use of a complex antigen such as living 

 bacteria, for there may be agglutins, bactericlysins, and sensitiz- 

 ing substances, all present in the cne immune serum. In view of 

 these facts, there is now no longer any room for doubt concerning 

 the important idle of the blood fluids as apart from the phago- 

 cytes. 



An orderly arrangement of our subject now calls for some con- 

 sideration of the offensive powers of the enemy. In what does the 

 virulence Cif bacteria consist ? 



Factors in Virulence. 



The mere implantation of bacteria on or in the animal body. is 

 not sufficient of itself to produce infection. The number of germs 

 used, in other words, the dose ; the path of entrance, he it digestive 

 tract, respiratory, or other mucous membranes, sound skin, or 

 into a v^ound ; the condition and constitution of the animal; all 

 these are factors of importance. But, with these conditions 

 remaining the same, different bacteria show all grades of viru- 

 lence or ability to attack with success. 



In the cac-e oi a number of organisms there is the power to 

 secrete toxins : tetanus, diphtheria, and botulinus being examples. 

 Grown in broth cultures and filtered, the filtrates are intensely 

 toxic by injection, or, in the case of the last-named, by ingestion, 

 in all three cases after an incitbatioti period, and, further, it is 

 remarkable <-hat the bacteria themselves, filtered off and washed, 

 contain only slight traces of the specific poison. Two points of 

 interest arise in this connexion, viz., the incubation period which 

 occurs before symptoms develop ; and the question as to the inclu- 

 sion of these organisms in the class of parasites or of saprophytes. 

 The incubation period has been explained as the interval required 

 for absorption of the toxin by the particular vulnerable cells, and 

 in the case of tetanus absorption appears to take place along the 

 nerve trunks until cell 3 of the C.N.S. are reached. Bacillus 

 botulinus is certainly a saprophyte, for it is non-pathogenic, grow- 

 ing and secreting its toxin in dead organic matter, v.v.d being 

 responsible for numerous cases of food poisoning. 



