April 1, 1891.1 



KNOWLEDGE 



77 



sheep, kills a guinea-pig, and then yields bacilli that are 

 fatal to sheep. Exposure to a temperature of 55° C, or 

 treatment with -5 to 1 per cent, carbolic acid, deprives the 

 bacilli of their virulence. The virulence of the bacillus 

 is also altered by passing the bacillus through different 

 species of animals. The bacillus of sheep or cattle is 

 fatal when re-inoculated into she^p or cattle ; but, if 

 inoculated in mice, the bacilli then obtained lose their 

 virulence for sheep or cattle ; only a transitory illness 

 results, and the animals are protected for a time against 

 virulent anthrax. The possibility of mitigating the virus 

 depends upon the species of animal ; rodents cannot be 

 rendered immune by any linown " vaccin." The same 

 process is employed by M. Pasteur in his now celebrated 

 inoculations for hydrophobia. In the course of his expe- 

 riments some very curious facts have come to light ; it 

 has been discovered that " passing the virus through 

 various animals considerably modifies its properties. By 

 inoculating a monlcey from a rabid dog, and then passing 

 the virus through other monlieys, the virulence is dimi- 

 nished ; but by inoculating a rabbit from the dog, and 

 passing the virus from rabbit to rabbit, the virulence is 

 increased." In swine-erysipelas Pasteur and Tliuillier 

 discovered that "by passing the virus through pigeons 

 the wulence was increased, but by passing it through 

 rabbits it was progressively diminished. Thus a virus 

 was obtained from a rabbit, which produced only a mild 

 disease in pigs, and after recovery complete immunity.'' 



To the non-professional observer, these facts seem 

 pregnant with deep meaning ; they appear to point to the 

 change from the old inoculation for small-pox to the 

 vaccination of Jemier, in which a similar but less virulent 

 disease may be communicated from a lower animal to man, 

 producing immunity from the graver disease ; and as we 

 look down the long lists of bacilli, some of deadly vu-u- 

 lence and some innocuous, yet all belonging to the same 

 group of germs, we wonder whether eventually it may not 

 be foimd tliat a cultivation of innocent germs may be 

 made to supplant the more malignant forms ; and this 

 opens up the whole question of immunity, which forms 

 one of the most interesting portions of Dr. Crookshank's 

 book. 



Immunity may be natural or acquired. We all know 

 that certain individuals are much more subject to infec- 

 tious diseases than others, even of the same family. In 

 some diseases one attack renders the person impervious to 

 the same disease ; but this is not always the case, some- 

 times after a time the protective influence of the first 

 attack ceases, and the individual succumbs to the disease 

 a second time. In certain diseases one attack predisposes 

 to a recurrence of tlie disease, as for example erysipelas ; 

 and again, " the occurrence of one disease is stated to 

 induce a liability to others; small-pox and typhoid fever 

 are regarded as predisposing to tuberculosis." When 

 Pasteur first began to try to mitigate the virulence of 

 anthrax, he found that by cultivating the microbe in 

 chicken broth, and allowing it to remain for several 

 months before carrying on successive cultivations in fresh 

 media, " the new generations which were then obtained 

 were found to have diminished in virulence, and ultimately 

 a virus was obtained which produced only a slight dis- 

 order ; and on reco\ery the animal was found to be proof 

 against inoculation by virulent matter." This change in 

 the quality of the virus M. Pasteur attributed to a pro- 

 longed contact with the oxygen of the air, and he shows 

 that if the cultivation of these germs is carried on in 

 sealed tubes, admitting very little air, the virulence is 

 retained. Heat also has been found to diminish the viru- 

 lence of the bacillus of anthrax, and the same has been 



brought about by chemical means, carbolic acid in minute 

 quantities, and bichromate of potash added to a cultivation, 

 " gave after three days a new growth, which killed rabbits, 

 guinea-pigs, and half the sheep inoculated; after ten 

 days rabbits and guinea-pigs, but not sheep ; and after a 

 longer time even guinea-pigs were unaffected." 



Tin discussing the question of what constitutes immunity, 

 Dr. Crookshank gives some very curious and suggestive 

 facts. He says ; " Kaulin has shown that Axinrtiillus niijer 

 develops a substance which is prejudicial to its own 

 growth in the absence of iron salts in the nutrient soil. 

 Pasteur has suggested that in rabies, side by side with 

 the living and organized substance, there is some other 

 substance which has, as in Raulin's experiment, the 

 power of arresting the growth of the first substance. If 

 we accept the theory of arrest by some chemical substance, 

 we must suppose that in the acquired immunity afforded 

 by one attack of an infectious disease this chemical sub- 

 stance is secreted, and, remaining in the system, opposes 

 the onset of the micro-organism at a future time. In the 

 natural immunity of certain species and individuals we 

 must suppose that this chemical substance is normally 

 present." 



Passing over two other theories, each of which presents 

 certain difliculties, we find the curious fact that in some 

 cases the white blood-cells appear to have the singular 

 power of destroying bacteria. " If anthrax bacilli are 

 inoculated in the fi.-og, the white blood-cells (leucocytes) 

 are observed to incorporate and destroy them until they 

 entirely disappear, and the animal is not aflected. But if 

 the animal, after inoculation, is kept at a high tempera- 

 ture, the bacilli increase so rapidly that they gain the 

 upper hand over the leucocytes, and the animal succumbs. 

 In septicipmia of mice the white blood-cells are attacked and 

 disintegrated by the bacilli in a similar way. It is, however," 

 adds Dr. Crookshank, " difficult to accept any explanation 

 of immunity from these observations — to suppose, for 

 example, that immunity depends upon the micro-organisms 

 being unable to cope with the leucocytes in certain species. 

 It is difficult to conceive that the leucocytes in the blood 

 and tissues in the field-mouse are differently constituted 

 ft'om those in the house-mouse, so that they form an 

 effectual barrier in the one case, though so readily 

 destroyed in the other." Hence we imderstand that 

 field-mice are exempt frorc the septictemia which is fatal 

 to house-mice and sparrows, the representative bacilli of 

 the disease being found most commonly in the interior 

 of the white blood-corpuscles. Pei'haps the immunity of 

 field-mice may result from some chemical secretion or the 

 difference in their mode of hfe and in their food. This 

 is a subject still open to investigation. 



In anthrax "a drop of blood from an aficcted animal, 

 or a minute portion of a cultivation, introduced under the 

 skin of a mouse or guinea-pig, causes its death, as a rule, 

 in from twenty-four to forty-eight hours. Sheep fed upon 

 potatoes which have been the medium for cultivating the 

 bacillus, die in a few days. Goats, hedgehogs, sparrows, 

 cows, horses are all susceptible. Rats are infected with 

 difficulty. Pigs, dogs, cats, white rats, and Algerian sheep 

 have an immunity fi-om the disease. Frogs and fish have 

 been rendered susceptible by raising the temperature 

 of the water in which they lived." In this list we 

 find the same difference with regard to sheep as in the 

 two species of mice. To the common sheep the disease 

 is fatal, whilst the Algerian sheep is immune. This 

 immunity of certain species has been seized upon by 

 agriculturists, who, when their flocks and herds suffer 

 from a certain disease, have found it beneficial to change 

 the breed. At the Cape of Good Hope, for instance, many 



