own secretion, as well as to a certain degree against various sickening external influences.
Prominent naturalists are at present occupied in inquiring for a reasonable way of interpreting the causes of sickness and the conditions of immunity from it, or the resistance offered by a sound organism. Sickness, as well as health, according to one of the prevailing theories, depends upon chemical causes, viz., on the presence and predominance of various complex substances generated in the juices and tissues of the body by unknown processes, in which bacteria may sometimes play an important part. According to another theory, the living animal cells are engaged in a continual struggle against intruding micro-organisms. Animal cells are considered as individuals similar in character to the order of Amœbæ, which are unicellular organisms of the class of Protozoa. Metschnikoff found that certain cells of the animal body are endowed with the faculty of swallowing and digesting intruding bacteria of every kind, harmless ones as well as pathogenic ones, or such as produce disease. Not all elementary organs of the body are equally qualified for this purpose, the function being intrusted to certain cells of the tissues and blood, which Metschnikoff calls phagocytes. Health as well as disease depends upon which party is victorious in the struggle. Health is insured as long as the cells are capable of overpowering the intruding bacteria; an animal in such a condition is secure against disease. Experiments performed by Metschnikoff have given evidence that the bacilli of splenic fever are easily devoured and digested by phagocytes. On the other hand, several observers of late have maintained that the liquid part of blood, the plasma, and even common albumen, possess the faculty of killing bacteria. This, however, appears improbable, and a final decision of the question has still to be expected in future.
Susceptibility to diseases is as variable as sensitiveness to vegetable and animal poisons. Judging from the current opinion that putrefying animal matter is the principal bearer and transporter of infectious germs, we are forced to ascribe a high degree of immunity to certain animals which, like swine, ducks, chickens, and rats, are accustomed to select their food from places where such matter is accumulated. Predisposition for splenic fever is stronger among herbivora than among carnivora; birds of prey seem to be quite free from it. Experiments on sheep, performed by Pasteur, the results of which were confirmed by application on a large scale, gave evidence that immunity against splenic fever may be acquired by systematic inoculation of the attenuated virus very much as small-pox is prevented by vaccination.
Various herbivora, chiefly horses, sheep, and goats, are exposed to a disease called "glanders," which ends by death in most cases.