When it was first suggested that diseases were due to the action of minute organisms, the question as to the mode of their action naturally arose, and various theories were promulgated. Some authorities were disposed to refer the deleterious influence of the microbes to the mechanical blocking-up of the vessels by accumulation of large numbers. This view, however, was obviously insufficient to account for the phenomena which were observed. A more reasonable view refers their baneful effects to a fermentative process which it would appear to be their particular function to originate. Some experiments which were made with reference to septic infection by Dr. Hiller went far to prove that the bacteria themselves were perfectly harmless, but were capable of inducing chemical changes in the fluids and solids of the body which led to the formation of animal poisons. Dr. Burdon Sanderson, in commenting on this function of bacteria, and particularly on Dr. Hiller's experiments, remarks that having collected a considerable mass of bacterial material, that is, of bacteria obtained from various fluids in advanced putrefaction, on a filter, Dr. Hiller washed the mass, just as one washes a precipitate, a great number of times; then diffused the material which had been so washed in distilled water, and injected it in repeated doses into the circulating blood of animals. The injections were entirely without effect. Hiller next proceeded to inoculate himself with the same material, and again without effect. The advocates of bacteria at once objected to Hiller's experiment that the bacteria, not being accustomed to distilled water, were so injured by the repeated washings that they had lost their activity. The criticism, however, might just as well have been spared, for it afforded Hiller the opportunity of proving by experiment, which was, of course, easy enough, that the washed bacteria were as lively and as capable of development as ever. It would appear from these observations as to the function of bacteria in the production of septic infection that they are really manufacturers of poison, and that when freed themselves from the material in which they live they are perfectly innocent. This view, which was promulgated ten years ago, is the one which is in favour with advanced pathologists, and it is a remarkable development of Dr. Hiller's researches that bacteria are now constantly employed for the purpose of producing toxic fluids, which are used for purposes of diagnosis and even of curing disease, as illustrated by the present use of tuberculin for the detection of tuberculosis, mallen for the discovery of glanders in horses, a preparation of the bacillus of diphtheria, modified by passing through the system of the horse, for the cure of diphtheria in man, and the use of the modified cultivation of the bacillus of tetanus for the cure and prevention of that disease both in the higher and lower animals.

Fig. 75 B. Spirillum undula.

Fig. 75 B. Spirillum undula. 1 Chromatic granules. 2 Sap vacuoles. 3 Protoplasm. C. Spirillum rubrum with polar cilia.

The action of microbes in the production of contagious diseases was demonstrated most perfectly by Pasteur in his researches on chicken cholera and splenic fever (anthrax). In the course of these experiments he proved not only that pure cultivations of the microbe outside the body would produce the disease with absolute certainty in healthy fowls, but also the still more important fact that by modifications in the method of growing the organism the activity of the poison would become diminished until it finally ceased. This discovery has been taken advantage of largely by bacteriologists, with the result of ascertaining that an attenuated or weaker virus may be obtained not only by modifying the method of cultivation in certain media, but by passing the virus through the system of an animal belonging to a different class from the one originally attacked. Thus the bacillus of anthrax, after being passed through the guinea-pig, loses its fatal activity on cattle (Sanderson and Duguid); the bacillus anthracis of whatever source, after having been passed through the white mouse, loses its fatal activity on sheep (Klein and Roy); and the same organism, when passed through the South American rodent biscaehia, loses its fatal activity on cattle (Roy). That this weakening or loss of virulence does not depend upon the death of the bacilli is proved by the fact that if they are again cultivated in the ordinary way in nutritive media they recover all their former quality - power to injure and to kill.

It has been asserted that there are at least three micro-organisms which are without any pathogenic property, and which may, when grown under certain conditions, acquire such properties. Thus the common hay bacillus (bacillus subtilis), according to Buchner, may by cultivation be transformed into bacillus anthracis; a common bacillus which is present in the atmosphere may assume distinct pathogenic properties if grown in an infusion of the seeds of abrus precatorius, and the common mould aspergillus, when grown on alkaline material, assumes poisonous properties according to Grawitz. That these statements are absolutely without foundation may be positively asserted on the evidence of numerous experiments; and it may be affirmed, on the contrary, that in no case does an innocent or benign organism acquire the power to excite a specific form of disease under any method of cultivation which has yet been devised.

The specific organisms which are found in contagious disease to which the horse is subject, are referred to and illustrated in the description of those affections.