4 Veterinary Medicine. 



and the chicken its apoplectiform septicaemia. Acquired immunity 

 is equally familiar as shown in all cases of recovery from a mi- 

 crobian disease, and in the fact that after a first attack of many 

 such diseases there is an acquired resistance to a second attack 

 for a greater or less length of time. This antagonism is in 

 degree only, and too often an overdose, or a specially virulent 

 or potent germ, or a particular condition of the system exposed, 

 will cause the latter to succumb to a microbe, to which it would 

 otherwise have proved immune. In general terms it may be 

 said that, as the microbe and its toxins are the main vulnerant 

 agents in establishing the disease, so the body cells (leucocytes, 

 erythrocytes, tissue. cells) and their toxins (antitoxins) are the 

 resistant or immunizing agents. It is an antagonizing of specific 

 pathogenic agents by defensive substances derived from the 

 normal body cells. The attack and the repulse are each made 

 through the soluble chemical products of the living organism 

 (microbe or body cell), and such products are diffused through 

 the serum or lymph. In this may be included the liquid 

 contents of the body cells for when these englobe the invading 

 microbe, the latter, until it perishes, continues to produce its 

 pathogenic products, which are the more concentrated and 

 effective because they are in the main confined within the nar- 

 row limits of the cell. Hence many cells are destroyed in the 

 clash of forces, and the result wavers in the balance until de- 

 cided by the relative numbers or potency of the contending bands 

 of microbes and cells. 



Immunization, by products of the disease to be protected 

 against, is initiated by one of two methods : ist by the use of 

 enfeebled germs which will no longer prove fatal to the animal 

 {protective infection'): and 2nd, by employing the soluble toxic 

 products of the pathogenic germ in the al^sence of that organism 

 itself {protective intoxication'). Since however the real immediate 

 factor, which operates on the animal cell to cause the formation 

 of the defensive material, is the soluble chemical product of 

 the microbe, it follows that the success in both cases is due to the 

 same cause. The practical difference is that whereas the en- 

 feebled microbes are still vital, and capable of reproduction, they 

 may, under given favorable conditions, start up a new infection 

 which may propagate itself indefinitely as an epizootic ; the same 



