SOHIZOMYCETES AND DISEASE. - The presence of Schizomycetes in the blood, tissues, or organs of animals and man suffering from certain specific diseases is admitted, and has naturally suggested the question - Are they accompaniments only or have they any causal relations to the diseased conditions ? Their constancy in given cases excluded the former view. Next arose the discussion as to how the causal connexion comes about and in what it consists, a discussion which is still going on as to the details. The chief points now established may be expressed generally somewhat as follows.

In a given specific disease, due to the action of a definite Schizomycete, the latter may be conceived to be injurious in several ways. If it robs the blood or tissues of oxygen or of any other valuable constituent, or if its activity results in the excretion of poisonous substances or in their formation as products of degradation of the matrix, or if it simply acts more or less as a mechanical obstruction or irritant, - in any of these cases harm may result to the delicately adjusted organism of the host. It being known that Schizomycetes act thus in nutrient pabula outside the body, their rapid growth and multiplication inside can of course only be explained as due to their success in the pabula there met with, and are indications that they produce changes there which must result in abnormality so far as the host is concerned. This does not end the matter, however. The living tissues of a healthy animal exert actions which are antagonistic to those of the parasitic invader ; and it is now generally admitted that the mere admission of a Schizomycete into an animal does not necessarily cause disease. Were it otherwise it is difficult to see how the higher organisms could escape at all. Schizomycetes abound all over, about and around us ; many, of course, are unable to live in the fluids of the body, but many are able to do so. Something must therefore be placed to the action of the tissues of the host, which when healthy can " resist " the attempts of a Schizomycete to settle, grow, and multiply with fatal effect. Much can undoubtedly be explained by this struggle for existence between the cells of the parasite and those of the healthy tissues invaded. But the higher organisms, again, present obstacles of other kinds to the lodgment of Schizomycetes : ciliary actions, active excretions, isolating processes of tissue-formation, he., may be mentioned. Thus not every Schizomycete met with in the body can do harm.

But even when a Schizomycete has gained access to the blood-vessels, lymph-passages, he., and has succeeded in establishing itself and multiplying, there are other facts to be taken into account before we dismiss the question as to its relations to disease. The rapidity of its growth may vary according to many circumstances, - temperature, oxidation, &c., - as well as the still partially obstructive action of the invaded organism ; whether the parasite excretes a poison, or simply robs the host, or distributes injurious agents of any kind, it is clear that everything which favours it aids in intensifying its action. And this may be local or general also according to complex circumstances. Of course sores, open wounds, &c., may render the access of a given Schizomycete very easy, and pave the way for its success in the tissues, &c., different strata of which may be exerting less and less resistance to its attacks. The study of this subject has led to the methods of modern surgery devised by Lister. It may be mentioned that Schizomycetes which produce bad effects on injured or dead tissues of wounds are not necessarily able to live in the healthy organism, however deadly the poisonous products of their action may be when they succeed in establishing themselves.

All these and many other facts, then, point to the conclusion that the mere presence of a Schizomycete in an organ or tissue is not sufficient proof of its causal relation to disease, and lead us to the following requirements to be satisfied before any such relation can be admitted (Koch) : - (l) given a specific disease in which a definite Schizomycete is constantly detected, and with a constant disposition with respect to the tissues, organs, &c., - this organism should be absent from animals free from the disease ; (2) the Schizomycete should be cultivated in nutrient media outside the body, kept pure for several " generations," and obtained in some quantity by these means ; (3) inoculation of a small amount of this pure cultivation should reproduce the specific disease in a healthy animal ; (4) the same foreign elements as before should be clearly detected in the tissues of the now diseased subject, and in the same relations as before.

The satisfying of all these requirements is difficult, and the necessity of overcoming the difficulties has led to what may almost be termed a special branch of medical ait. At the same time the majority of the principles which are here becoming recognized have long been known to biologists, and especially to botanists, and there are still numerous indications of a want of botanical training on tItc-part of writers on these subjects. It is impossible here to even mention all the methods devised for staining, preparing, and examining tissues, be., and the Schizomycetes they contain, or for cultivating these minute organisms under constant conditions on sterilized potatoes, bread-paste, jelly, blood-serum, he., or in animal infusions or fluids, he. Some of the more important points in cultivation have already been referred to ; the literature must be consulted for further details.a (H. M. W.)