PATHOLOGY (from Gr. raBos, suffering)  , the science dealing with the theory or causation of disease . The term by itself is usually applied to animal or human pathology, rather than to vegetable pathology or Phytopathology (see PLANTS: Pathology) . The outstanding feature in the history of pathology during the 19th century, and more particularly of the latter half of it, was the completion of its rescue from the thraldom of abstract philosophy, and its elevation to the dignity of one of the natural sciences . Our forefathers, if one may venture to criticize them, were too impatient . Influenced by the prevailing philosophy of the day, they interpreted the phenomena of disease through its lights, and endeavoured from time to time to reduce the study of pathology to philosophical order when the very elements of philosophical order were wanting . The pathology of the present day is more modest; it is content to labour and to wait . Whatever its faults may be--and it is for our successors to judge of these—there is this to be said in its favour: that it is in nowise dogmatic . The eloquence of facts appeals to the scientific mind nowadays much more than the assertion of crude and unproven principles . The complexity and mystery of action inherent in living matter have probably been accountable for much of the vague philosophy of disease in the past, and have furnished one reason at least why pathology has been so long in asserting its independence as a science . This, indeed, holds good of the study of biology in general . There are other factors, however, which have kept pathology in the background . Its existence as a science could never have been recognized so long as the subjects of physics, chemistry and biology, in the widest acceptation of the term, remained unevolved .

Pathology, in fact, is the

child of this ancestry; it begins where they end . Progress in the study of pathology has been greatly facilitated by the introduction of improved methods of technique . The certainty with which tissues can now be fixed in the state they were in when living, and the delicacy Progress. with which they can be stained differentially, have been the means of opening up a new world of exploration . Experimental pathology has benefited by the use of antiseptic surgery in operations upon animals, and by the adoption of exact methods of recording; while the employment of solid culture media in bacteriology—the product of Koch's fertile genius—is responsible for a great part of the extraordinary development which has taken place in this department of pathological research . The discoveries made in pathological bacteriology, indeed, must be held to be among the most brilliant of the age . Inaugurated by Pasteur's early work, progress in this subject was first marked by the discovery of the parasite of anthrax and of those organisms productive of fowl-cholera and septic disease . Then followed Koch's great revelation in 1882 of the bacillus of tubercle (fig . 22, P1 . II.), succeeded by the isolation of the organisms of typhoid, cholera, diphtheria, actinomycosis, tetanus, &c . The knowledge we now possess of the causes of immunity from contagious disease has resulted from this study of pathological bacteriology: momentous practical issues have also followed upon this study . Amongst these maybe mentioned the neutralizing of the toxins in cases of diphtheria, tetanus and poisonous snake-bite; " serum therapeutics "; and treatment by " vaccines." By means of " vaccination " we are enabled to induce an active immunity against infection by certain pathogenic bacteria . The value of such protective inoculations is demonstrated in the treatment against small-pox (Jenner), cholera, plague (Haffkine) and typhoid (Wright and Semple) .

Pasteur's inoculation against

hydrophobia is on the same principle . " Vaccines " are also used as a method of treatment during the progress of the disease . Sir A . Wright and others, in recent work on opsonins, have shown that, by injecting dead cultures of the causal agent into subjects infected with the organism, there is produced in the body fluids a substance (opsonin) which apparently in favourable conditions unites with the living causal bacteria and so sensitizes them that they are readily taken up and destroyed by the phagocytic cells of tissues . Before the discovery of the bacillus of tubercle, scrofula and tuberculosis were regarded as two distinct diseases, and it was supposed that the scrofulous constitution could be distinguished from the tubercular . It was always felt, however, that there was a close bond of relationship between them . The fact that the tubercle bacillus is to be found in the lesions of both has set at rest any misgiving on the subject, and put beyond dispute the fact that so-called scrofulous affections are simply local manifestations of tuberculosis . A knowledge of the bacteriology of scrofulous affections of bone and joints, such as caries and gelatinous degeneration, has shown that they also are tubercular diseases—that is to say, diseases due to the presence locally of the tubercle bacillus . At a very early period it was held by Virchow that the large cheesy masses found in tuberculosis of the lung are to be regarded as pneumonic infiltrations of the air-vesicles . Their pneumonic nature has been amply substantiated in later times; they are now regarded simply as evidence of pneumonic reaction to the stimulus of the tubercle bacillus . The caseous necrosis of the implicated mass of lung tissue, and indeed of tubercles generally, is held to be, in great measure, the result of the necrotic influence of the secretions from the bacillus . Tubercular pneumonia may thus be looked upon as comparable to pneumonia excited by any other specific agent .

In the " seventies " of the 19th century feeling ran somewhat high over the

rival doctrines concerning the origin of pus-corpuscles, Cohnheim and his school maintaining that they were derived exclusively from the blood, that they were leucocytes which had emigrated through the walls of the vessels and escaped into the surrounding tissue-spaces, while Stricker and his followers, although not denying their origin in part from the blood, traced them, in considerable proportion, to the fixed elements, such as fibrous tissues and endothelia . Our present-day knowledge prompts the adoption of a middle course between the two theories . The cells found in an inflamed part are undoubtedly drawn from both sources, but while the blood leucocytes have a great tendency to become fatty and to die, those cells derived from the fixed tissues incline more to organization; the latter are, in fact, the source of the cicatrix which follows upon the cessation of suppuration (fig . 23, P1 . II. and figs . 31 and 32, Pl . III.) . Organization and healing have been keenly inquired into, with results which seem to point the lesson that all methods of healing are to be regarded as extensions of the natural phenomena of growth . Normal cytology, of late, has become a science of itself, and has had a direct bearing upon that which is pathological . At no time has so much been done to advance our knowledge of diseases of the nervous system as during the last thirty years of the 19th century . The localization of function in the cerebral and in the cerebellar cortex has doubtless been the main cause of this progress, and has proceeded pari passu with an extended insight into the structure and connexions of the parts concerned . The pathology of aphasia, as worked out by a combination of the experimental, the pathological and the anatomical lines of inquiry is a favourable example of what has been accomplished .

The origin, nature, and

propagation of neoplasms of all kinds, especially of those which are malignant, are engaging much attention . Much light has been thrown upon the functions and diseases of the blood-forming tissues . The origin of the corpuscles, previously a matter of so much difference of opinion, is now pretty fairly set at rest, and has proved the key to the interpretation of the pathology of many diseases of the blood, such as the different forms of anaemia, of leucocythaemia, &c . It is largely to researches on the bone marrow that we owe our present knowledge of the origin and the classification of the different cellular elements of the blood, both erythrocytes or red corpuscles, and the series of granular leucocytes or white corpuscles . Whatever be the ancestral cell from which these cells spring, it is in the bone marrow that we find a differentiation into the various marrow cells from which are developed the mature corpuscles that pass from the marrow into the blood circulation . The healthy bone marrow reacts with remarkable rapidity to the demand for more blood cells which may be required by the organism; its reactions and variations in disease are very striking . If the demand be for the red cells owing to loss from haemorrhage or any of the anaemias,the fatty marrow is rapidly replaced by cellular elements; this is mainly an active proliferation of the nucleated red cells, and gives rise to the erythroblastic type of marrow . If the white cells be required, as in local suppurating abscess, general septicaemia, acute pneumonia, &c., there is an active proliferation of the myelocytes to form the polymorpho-nuclear leucocytes, so that we have in this condition a leucoblastic transformation of the fatty marrow . The cytology of bone marrow, with the technique of blood examination, is of great assistance in the diagnosis of different pathological conditions . The deleterious influence of high blood-pressure has engaged the attention of physicians and pathologists in later years, and the conclusion arrived at is, that although it may arise from accidental causes, such as malcomposition of the blood, yet that in many instances it is a hereditary or family defect, and is bound up with' the tendency. to gout and cirrhotic degeneration of the kidney . The pathology of intra-cardiac and vascular murmurs has also been inquired into experimentally, the general impression being that these abnormal sounds result, in most cases at least, from the production of a sonorous liquid vein . Pneumonia of the croupous type has been proved to be, as a rule, a germ disease, the nature of the germ varying according to circumstances .

The structural changes occurring in the bronchi in catarrhal

bronchitis have also been ascertained, and, as in the case of pneumonia, have been shown to be frequently excited by the presence of a microphyte . The vexed question of the diagnosis of diphtheria is now a thing of the past . Quite irrespective of the nature of the anatomical lesion, the finding of the diphtheria bacillus on the part affected and the inoculability of this upon a suitable fresh soil are the sole means by which the diagnosis can be made certain . The part played by the thyroid body in the internal economy of the organism has also received much attention . The gland evidently excretes, or at any rate gets rid of, a certain waste product of a proteid nature, which otherwise tends to accumulate in the tissues and to excite certain nervous and tissue phenomena . It wastes in the disease known as " myxoedema," and the above product gathers in the tissues, in that disease, to such an extent as to give rise to what has been termed a " solid oedema." It is questionable if the substance in question is miicoid . The pituitary body probably subserves a like purpose . When the pancreas is excised in an animal, or when it is destroyed in man by disease, grape-sugar appears in the urine . The gland is supposed to secrete a ferment, which, being absorbed into the portal circulation, breaks up a certain portion at least of the grape-sugar contained in the portal blood, and so prevents this overflowing into the circulation in general . The transplantation of a piece of living pancreas into the tissues of an animal, thus rendered artificially diabetic, is said to restore it to health . Pathological chemistry has been remarkable chiefly for the knowledge we have obtained of the nature of bacterial poisons . Certain of these are alkaloids, others appear to be albumoses .

The publication of Ehrlich's chemical, or rather

physical, theory of immunity has thrown much light upon this very intricate and obscure subject . Pathology is the science of disease in all its manifestations, whether structural or functional, progressive or regressive . In times past it has been the habit to look upon its sphere connexion as lying really within that of practical medicine, and with human medicine more particularly; as something Biology . tagged on to the treatment of human disease, but unworthy of being studied for its own sake as a branch of knowledge . Such a view can recommend itself to only the narrowest of minds . A bearing, and of course an essential bearing on the study of medicine, it must always have . A system of medicine reared upon anything but a pathological basis would be unworthy of consideration . Yet it may well be asked whether this is the final goal to be aimed at . Our starting-point in this, as in all departments of biological study, must be the biological unit, and it is to the alterations to which this is subject, under varying conditions of nutrition and stimulation, that the science of pathology must apply itself . Man can never be the only object of appeal in this inquiry . The human organism is far too complex to enable us to understand the true significance of diseased processes . Our range must embrace a much wider area—must comprise, in fact, all living matter—if we are ever to arrive at a scientific conception of what disease really means .

Hence not only must the study of our subject include the diseases

peculiar to man and the higher animals, but those of the lowest forms of animal life, and of plant life, must be held equally worthy of attention . Modern research seems to show that living protoplasm, wherever it exists, is subject to certain laws and manifests itself by certain phenomena, and that there is no hard and fast line between what prevails in the two kingdoms . So it is with the diseased conditions to which it is a prey: there is a wonderful community of design, if the term may be used in such a sense, between the diseases of animals and plants, which becomes singularly striking and instructive the more they are inquired into . Utilitarian, or perhaps rather practical, considerations have very little to do with the subject from a scientific point of view—no more so than the science of chemistry has to do with the art of the manufacturing chemist . The practical bearings of a science, it will be granted, are simply, as it were, the summation of its facts, with the legitimate conclusions from them, the natural application of the data ascertained, and have not necessarily any direct relationship to its pursuit . It is when studied on these lines that pathology finds its proper place as a department of biology . Disease as an entity —as something to which all living matter is subject—is what the pathologist has to recognize and to investigate, and the practical application of the knowledge thus acquired follows as a natural consequence . Since pathology is the science of disease, we are met at the very threshold by the question: What is disease ? This may Health best be answered by defining what we understand by and health . What do we mean when we talk of a healthy Disease. organism ? Our ideas upon the subject are purely arbitrary, and depend upon our everyday experience . Health is simply that condition of structure and function which, on examination of a sufficient number of examples, we find to be commonest .

The term, in fact, has the same significance as " the normal." Disease we may define, accordingly, as any departure from the normal

standard of structure or function of a tissue or organ . If, for instance, we find that instead of the natural number of Malpighian bodies in the kidney there are only half that number, then we are entitled to say that this defect represents disease of structure; and if we find that the organ is excreting a new substance, such as albumen, we can affirm logically that its function is abnormal . Once grant the above definition of disease, and even the most trivial aberrations from the normal must be regarded as diseased conditions, quite irrespective of whether, when structural, they interfere with the function of the part or not . Thus an abortive supernumerary finger may not cause much, if any, inconvenience to the possessor, but nevertheless it must be regarded as a type of disease, which, trivial as it may appear, has a profound meaning in phylogeny and ontogeny . Classification.—From the foregoing it will be gathered that the problems in pathology are many-sided and require to be attacked from all points of vantage; and the subject falls naturally into certain great divisions, the chief of which are the following: I . Morbid anatomy . (a) Naked-eye or macroscopic . (b) Morbid histology or microscopic . II . Pathological physiology . IV . Aetiology .

V . Pathological chemistry . The term " pathogenesis " has reference to the

generation and development of disease, and that of " aetiology," in its present bearing, has to do with its causes . The use of the term " pathological physiology " may at first appear strange, for if we define physiology as the sum of the normal functions of the body or organism, it may be hard to see how there can be a physiology which is pathological . The difficulty, however, is more apparent than real, and in this sense, that if we start with a diseased organ as our subject of inquiry, we can quite properly, and without committing a solecism, treat of the functions of that organ in terms of its diseased state .