66 Pathogenic Bacteria. 



In the study of the action of bacterial poisons it was found 

 that, when a dose is injected insufficient to cause death, and yet 

 such as to lead to more or less disturbance of the bodily 

 functions, there results finally an immunity to the disease on 

 the part of the animal so inoculated. In such investigations 

 the method is to inject a small dose in the first instance and to 

 increase the dose subsequently until a dose is reached which, 

 without the preliminary inoculations, would certainly have been 

 fatal. The nature of the change which this series of inoculat ions 

 induces so as to establish the state of immunity is very imper- 

 fectly understood. One result, however, of great practical 

 importance is, that the serum of the blood is charged with 

 immunising power, and that this power can be conveyed to a 

 second animal by injecting into that animal some of the serum 

 of the first. This is the principle on which the modern treat- 

 ment of diphtheria is based. A horse is made in)nmne to the 

 poison of the diphtheria bacillus, and when this is successfully 

 carried out the immunity which the horse acquires enables it 

 to resist the effects of enormous doses of diphtheria poison. The 

 serum of its blood is then obtained and prepared for injection 

 into the tissues of children who have been attacked by the 

 disease. The child in this way gets at once the advantage of 

 the tedious process of acquiring the immunity to whicii the 

 horse has been submitted. Such a serum is called an antitoxic 

 serum. 



In regard to the bacteriology of Typhoid Fever one or two 

 general points of interest may be noted. The avenue of inlec- 

 tion by which the microbe reaches the body is the alimentary 

 canal. Here also occur some of the structural changes which 

 characterise the disease. The bacillus is discovered in the tissues 

 of various abdominal organs — viz., the lymph glands, the spleen, 

 and the liver. 



The bacillus has no very clear character by which it can 

 be at once distinguished from all other bacilli. It is very closely 

 allied to the other groups of bacilli, and from these it is a 

 matter of no small difficulty to distinguish it. The obscurity 



