26 VETERINARY LECTURES 



43. Of late years a great change has taken place in the mode of 

 treating derangement and diseases in farm stock generally, but more 

 especially those maladies that are now considered due to the presence 

 of pathogenic or disease-producing germs (bacteria and their products), 

 which gain access to the body by the medium of the food, water, 

 inhalation, and by inoculation through wounds, etc. Bacteriologists 

 and highly-trained scientific workers have, and are now preparing, 

 various serums and vaccines, which are injected hypodermically or 

 otherwise into the body, and are said to act by generating an active 

 immunity against the corresponding living disease-producing germs. 

 The introduction of vaccine prepared from a certain kind of bacteria 

 into the blood results in the production of certain anti-bacterial 

 material, called by Sir A. E. Wright 'opsonins' (opsono = to prepare 

 food for). By estimating the amount of opsonins in the blood we 

 realize the amount of phagocytosis which is going on, or, in other 

 words, the progress of immunization. Thus the natural immunity 

 of the body can be materially aided by administering appropriate 

 doses of suitable vaccines at various intervals. According to Wright, 

 the natural remedy of any bacterial disease is brought about by a 

 series of anti-inoculations and immunizing responses, the vaccine or 

 inoculative matter being derived from the infected or diseased 

 portions of the body, consisting of the bacteria or their products. 

 Each successive immunizing response leaves the body defences 

 more reliable, and finally complete immunity is produced. Some- 

 times, when the bacteria are poured forth in excessive numbers and 

 their toxins are unusually virulent, the immunization fails, and the 

 disease-producing germs multiply in the body. In such cases appro- 

 priate doses of vaccines injected at right intervals will stimulate the 

 production of the protective material, and bring about a cure. Of 

 course, it must be borne in mind that appropriate and suitable 

 vaccines must be used in each case. Thus the bacteria of disease 

 have been combated by fortifying the resisting power of the white 

 corpuscles of the blood (phagocytes), (pars. 422, 423, and 426), the 

 stimulus used being preparations from other bacteria ; but from 

 experiments it has been found that disease-producing bacteria have 

 been successfully resisted by chemical agents. It therefore becomes 



