102 Report S.A.A. Advancement of Science. 



Knowing the functions and behaviour of these cells in diseases 

 other than horse-sickness, we are justified from the above facts in 

 attributing to this class of white cell an essential part in the defence 

 of the system against horse-sickness. The highly resistent or hyper- 

 immune horse, which can tolerate enormous doses of infectious blood, 

 does not, however, shew any permanent increase in the number of 

 these cells, and further, the blood of such a horse could be abstracted 

 and replaced by the blood of a normal horse — all of these cells being 

 thereby theoretically removed — and still the animal would retain its 

 immunity or resistive power to the disease. The defensive mechanism, 

 therefore, of the immuned horse cannot be entirely in the white cells 

 circulating in the blood. 



Through the researches of Metchnikoff, however, we know of the 

 existence of other cells, the so-called " fixed cells," which, while 

 immobile and attached to the various organs and tissues in which 

 they are found, are nevertheless able to exert a defensive action 

 similar to that of the free or wandering white cells. When once 

 these cells — free and fixed — have become trained to take the 

 offensive against the microbe of horse-sickness (Metchnikoff 

 speaks of the " education " of the cell), the animal successfully 

 resists infection, and so becomes immune. When thus immunised, 

 successive reinfection, either natural or artificial, strengthens the 

 defensive mechanism, and increases the degree of immunity, to such 

 an extent that we are able, if we wish, to abstract the animal's blood, 

 and utilise this defensive and protective principle in the form of the 

 so-called immune serum. 



As far as we know, there is only one way in which the horse 

 can become actively immune, and that is by fighting and overcoming 

 the microbe of the disease. Natural recoveries from Horse-sickness, 

 hovv-ever, are not at all so rare an occurrence as we have in the past 

 supposed. When the disease has progressed as far as the stage of 

 effusion into the lung and heart sacs, recovery is, indeed, a forlorn 

 hope, but I am convinced that thousands of horses are annually 

 infected with horse-sickness, and recover .from the same, without 

 raising the suspicion in their owner's minds of the existence of the 

 disease. In fact, the horse is quite able to " put up a fight," as 

 the saying is, against horse- sickness, and often to win the battle 

 through the instrumentality of that best of physicians, the vis^ medic- 

 atrix naturae. Where, however, the infected animal is handicapped 

 by debility, bad management, hard work, or excessive exercise, the 

 disease becomes established in its familiar and recognised phases, 

 and when matters have gone thus far, vain is the help of man. 



If natural attacks and recoveries are frequent, it would seem 

 that no great difficulty should be met in securing recovery from 

 experimental infections.' Such, however, is not found to be the case 

 in the horse with his very varying degrees of susceptibility, and 

 hitherto the introduction of even minute doses of virus, if sufficient 

 to evoke a reaction at all, have been attended by the gravest risk of 

 the disease " getting out of hand," so to speak, and impenlhng the 



