Disease is the correlative of health, and the word is not capable of a more penetrating definition. From the time of Galen, however, it has been usual to speak of the life of the body either as proceeding in accordance with nature (κατά φvσι,v, secundum nal·uram) or as overstepping the bounds of nature (παρά φvσιv, praeter naturam'). Taking disease to be a deflexion from the line of health, the first requisite of medicine is an extensive and intimate acquaintance with the norm of the body. The normal condition of the body is capable of being determined without ambiguity; it is the absence from its structures and functions of every disease hitherto known. The structure and functions of the body form the subject of anatomy and physiology.

Physiology is, strictly speaking, the science of that which is κατά φvσιv, or secundum naturam, and it is usual to say that the theory of diseases is based upon physiology. But, although all that was implied in the Hippocratic term φvσις (natιιra) may be claimed as the subject-matter of physiology, yet, in the ordinary connotation of the term, physiology divides the empire with anatomy. To physiology the functions of the body are usually assigned, and to anatomy its form and structure. But, as a matter of fact, the structures and functions of the organism are not separable; structure is correlated to function, whether active, dormant, or extinguished, and in like manner function is the twin notion of structure. In the ultimate analysis neither term means anything without the other, and both together mean life. It is owing mostly to its name that physiology is supposed to have a preponderant interest for the theory of disease; the word anatomy is not well adapted to carry its own half of the structure* and-function dualism. Both in the historical development and in the logical connotation, anatomy is as much associated with the living and moving body as physiology itself; but its etymology has always been against it, and it has become more and more difficult to retain for anatomy anything beyond the technicalities of the dissecting-room. The subject of general anatomy has for the most part disappeared from modern text-books, its place being taken by histology, which deals with the minute structure of the simple tissues, and, in a wider acceptation, with the finer anatomy of all the organs and parts of the body. Histology, like anatomy, has had a somewhat technical or descriptive rôle assigned to it; and it is now mainly under physiology that the processes, activities, or living mechanisms of the body fall to be considered. The development of the body as a whole, and of its several tissues and organs, forms the subject of embryology; many of the physiological types of diseased processes, especially the cellular, are discoverable in the embryological period. For the-period of development, no arbitrary separation has been attempted hitherto between structure and function, and embryology is, in theory at least, as much physiological as anatomical. The development of function is a legitimate and even desirable subject of scientific study, and a more distinctive place is probably awaiting it in the future; but so indissoluble does the union of structure and function present itself in the period of genesis and growth that the function has hardly as yet come to be abstracted from the structure, or the structure from the function.

The theory of disease rests, therefore, upon physiology, with its more or less technical adjuncts. Pathology is all that physiology is, with the engrossing and difficult element of perturbation, deflexion, or shortcoming added. By virtue of this element of deviation from the line of health, pathology is a discipline apart, with an abundant literature of its own, and with separate academical institutes and chairs. But pathology is also a discipline apart by virtue of concepts proper to itself. A great part of the theory of disease deals with changes or defects of structure and perturbations or failings of function, which may be intricate or difficult to analyse, but are still well within sight of the line of health. Such are the common diseases of the organs and systems—the inflammations, catarrhs, degenerations, hypertrophies, and functional derangements without lesion of the respiratory, circulatory, nervous, genito-urinary, locomotor, and cutaneous systems. Constitutional or general diseases belong also to the province of perturbations from the physiological course,—such diseases as chlorosis, leukaemia, diabetes, gout, rheumatism, scurvy, rickets, Addison’s disease, exophthalmic goitre, and the febrile state. Again, congenital deficiencies or malformations, non-cancerous tumours, and the repairing of injuries exemplify no other laws than those of development and growth.

But with those examples the catalogue of physiological diseases is exhausted. We are left with a vast residue of diseases, which have always bulked largely in the popular mind, and have carried the most terrible associations with them. Such are the pestilences or diseases of peoples: — the plague, sweating sickness, cholera, yellow fever, typhus fever, relapsing fever, typhoid fever, diphtheria, small-pox, measles, scarlet fever, influenza, dengue. Such also are the cancers, consumptions, leprosies, and other loathsome infections. This enormous residue is more than the half of disease, and the definition of disease or the scheme of pathology is brought to a test in finding room within its scientific categories for such maladies as those. The popular imagination in all countries has personified them; medicine in its metaphysical period has regarded them as entities or things in themselves; and it remains to be seen in what way or to what extent medicine in its scientific period will bring them within the category of perturbations of the physiological life.

In considering, for a moment, where to place cancer in the pathological scheme, we shall arrive at a point of view from which the relation of the acute and chronic infections (or contagions) to diseases of the physiological order may be contemplated at least provisionally. Taking cancers, in a generic sense, to mean tumours that have acquired or are possessed of malignancy, we find that such tumours have many points in common with simple tumours,—that they have grown out of the tissues of particular organs or parts under particular (functional) circumstances, and that they may, in general terms, be traced back to that point at which they left the line of health (see Pathology). The tracing back of tumours along the physiological track is often difficult and laborious; but there is no tumour of the body whose origins are not at length discoverable within the limits of physiological action. That which makes any tumour a cancer is something over and beyond; it is a remarkable acquired property of reproducing its structure in manifold copies, or of infecting the organism of which it is itself a part. The tumour thus becomes a semi-independent power within the body; it may be said, in a political figure, to have acquired autonomy, or to have become imperium in impei'io. A due consideration of such a phenomenon as the infectiveness or cancerousness of some tumours will satisfy one that there are concepts in pathology which carry the investigator entirely beyond physiological bounds or out of sight of the line of health, which bring him face to face with the notion of a disease as a thing in itself, and which thus constitute a peculiar subject-matter. There is nothing that we know among biological phenomena altogether analogous to the semiindependence which an integral part of the body, or condition of the body, manifests towards the organism as a whole, and that, too, strictly in respect of its acquired devious or rebellious habit. The familiar definition of disease, morbus est vita praeter naturam, which embodies the notion of divergence from the line of health, makes no provision for an acquired autonomy of a morbid state; and that definition has to be supplemented by another, which will recognize the possibility of a disease becoming a thing in itself. The old definition of Van Helmont, morbus est ens reale subsistens in corpore, appears to satisfy the requirement; but that definition, although it grew out of the phenomena of disease as observed in fevers, was made too general, and has now associations that are too exclusively ontological and metaphysical. The supplementary definition should be as far as possible in the terms of the principal definition; and we shall provide best in the pathological scheme for such a disease as cancer if, in addition to tho formula morbus est vita prseter naturam, we construct a secondary formula, morbus est vivum in vivo.

The notion of autonomy acquired by a morbid state implies, naturally, a pre-autonomous stage of the disease, which had been a mere perturbation of the norm of tho body, capable of being measured by the physiological standard. The autonomous stage and the pre-autonomous stage, which may be demonstrated, in individual cases, for cancers, are a philosophical necessity for all other infective diseases that are marked by morphological features, or by structural characters rooted in and growing out of the proper textures of the body. Thus the peculiar skin eruption of small-pox, which is communicable from person to person, along with a distinctive course of fever, must have had pre-autonomous antecedents (not altogether historically vague) in certain casual conditions of the skin and associated constitutional disturbance, which had recurred and become inveterate, and had so attained to a degree of individuality or a point of autonomy at which they began to be propagated as an organic unit. Again, a second group of infections, exemplified by glanders, bovine tubercle, and syphilis, are rooted in deeper textural processes, which must have been at one time (and may still be) set up by the casual operation of ordinary causes, and at length became the occasion of infective mimicry. It is not so easy to picture (and it is not difficult, with a modern dominant school, to ignore) the casual morbid conditions or ordinary physiological perturbations out of which powerful infections like cholera, typhoid fever, or yellow fever may have arisen; but if the rise and consolidation of their autonomy be a subtle or even untraceable history, yet there are diseases, such as dysentery and erysipelas, which are apt to occur both as casual or spontaneous conditions and as specific infections side by side. Ophthalmia is an example of a purulent catarrh which is constantly arising de novo in Egypt from local causes in a non4nfective manner, and yet has become, on at least one memorable occasion, a powerful and widespread infection for British troops returning from that country and for the home garrisons for many years subsequently. Infective pneumonia in cattle, and more rarely in man, is an analogous case. In such an episode we observe the actual rise of the disease-autonomy. Again, all the infective diseases have degrees of intensity, at one extreme of which there must occur the vanishing point of their infective property; and those gradations of infectiveness are nowhere more noticeable than in the relation of cholera to choleraic diarrhoea. Further, the remarkable group of climatic fevers are not communicable from person to person (see Malakia) ; in that respect, and for the reason that the liability of the patient is anything but exhausted by one attack, they are examples of fevers without autonomy. There is not one of the infections that may not be profitably studied from the point of view of its autonomy, and of its more or less obscure pre-autonomous stage. That is a point of view from which even the pestilences and other specific diseases may be regarded as coming within the physiological categories. The large residue of diseases, which are more than perturbations of the physiological life, may still be joined by natural descent to the class of simple perturbations, if we can show for them how their autonomy was acquired, or what was their origin as diseasespecies.

There is an established place in the history of medicine, and there ought therefore to be room in the definition of <lisease, for epidemic outbreaks of purely psychical diseased states, such as the dancing madness (Tanzwuth), and the boys’ crusades; the epidemic diffusion of such morbid states is best approached from the point of view of an acquired autonomy (fixed idea) and an infective mimicry.

The physiological definition of disease, morbus est vita prater naturam, affords no place for parasitic diseases. However, the supplementary formula that has been proposed to meet the case of diseases existing autonomously in the body, morbus est vivum in vivo, will meet the case of parasitic diseases also. According to many pathologists of the present generation, the whole class of pestilences, fevers, and specific infections generally are caused by certain species of minute parasites invading the body; according to one form of that hypothesis the distinctive characters or specific marks (morphological and other) of those diseases are neither more nor less than the appropriate effects wrought upon the textures and fluids of the body by the respective species of parasites. In this way the great group of infective diseases, which are apt to be the stumbling-block of a scientific definition and logical scheme of disease, are easily disposed of by placing them beside the otherwise insignificant group of parasitic diseases. Whether all or any of those diseases are due in a sense to the invasion of parasites, or wholly caused by parasites, are questions that naturally fall to be settled by a careful sifting of a mass of evidence which has already proved to be peculiarly rich in opportunities for mistake. It may be expected that the facts of infective parasitism and the facts of acquired disease-autonomy will in the end find their place in a common theory of specific diseases, which might be expressed in terms of the physiological formula morbus est vita praeter naturam with the rider morbus est vivum in vivo.

The theory of remedies, which forms the second division of the science of medicine, is chiefly based upon pharmacology or toxicology. If pharmacology be considered as not co-extensive with toxicology, it will be taken tobe in great part pharmacographia, or the systematic description of articles of the materia medica—their source, preparation, physical properties, and the like. Toxicology is in its general sense the investigation of the physiological action of drugs, a science which is largely dependent upon experiments on the lower animals; in a more technical sense toxicology relates to the effects of poisons and the art of detecting them (see Poisons). The physiological action of drugs is the key to their therapeutical action. Therapeutics has been defined as “the discovery of the means by which a system of forces competent to eliminate any given perturbation may be introduced into the economy.” The adaptation of remedies to diseases is, however, greatly wanting in precision, and continues to be in large part empirical and traditional. It may be objected to the above definition that all diseases are not reducible to the category of “perturbations,” and that there is a certain scientific justification for the doctrine of specifics. Besides the articles of the materia medica proper, agencies such as electricity, baths, sea-voyages, and changes of climate generally, enter into the consideration of therapeutics, and two of those form the subject of special departments, viz., electro-therapeutics and hydropathy. Regimen and diet are also important factors in the treatment of disease; according to a contention of Hippocrates, it was in the dietetic needs of mankind that the medical art had its origin.

Subdivisions of Medicine as an Ai t and, Discipline.— The medical art (ars medendi) breaks away at once from the unity of the theory of disease. While there is but one body of pathological doctrine for either sex, for every period of life, and for every region and part of the organism, the practical art divides itself into departments and subdepartments. The most fundamental division is into internal and external medicine, or into medicine proper and surgery. The treatment of wounds, injuries, and deformities, with operative interference in general, is the special department of surgical practice (the corresponding parts of pathology, including inflammation, repair, and removable tumours, are sometimes grouped together as surgical pathology); and where the work of the profession is highly subdivided, surgery becomes the exclusive province of the surgeon, while internal medicine remains to the physician. A third great department of practice is formed by obstetric medicine or midwifery, and with obstetrics there is usually associated gynaecology, or the diseases peculiar to women. Diseases of children are the subject of a voluminous separate literature. Dermatology (diseases of the skin) is an important province of practice which, like the diseases of women and children, pertains as much to medicine as to surgery. The greatest of the so-called special departments of practice is ophthalmology (diseases and injuries of the eye). Laryngology is a department that owes its existence mainly to the invention of the laryngoscope, its special province being the treatment of the inflammations (ordinary and specific), tumours, and the like, to which the larynx is liable in common with other parts. Diseases of the ear (otology) form even a more restricted department of practice, owing to the comparative inaccessibility of the chief part of the organ of hearing. The congenital condition of deaf-mutism may or may not be taken as falling within the province of the last-mentioned subdivision. Dentistry or odontology is extremely limited in the range of its subject-matter; but it affords great opportunities for refinements of technical skill, and it is given up to a distinct branch of the profession.

The care of the weak-minded and the insane (psychological medicine) is an integral part of medical practice, inasmuch as it is concerned with diseases of the nervous system and with numerous correlated states of other organs; but it occupies a unique place by reason of the engrossing interest of the subjective phenomena. Habitual drunkenness is also a subject of special treatment.

' A state of war, actual or contingent, gives occasion to special developments of medical and surgical practice (military hygiene and military surgery). Wounds caused by projectiles, sabres, &c., are the special subject of naval and military surgery; while under the head of military hygiene we may include the general subject of ambulances, the sanitary arrangements of camps, and the various forms of epidemic camp sickness.

The administration of the civil and criminal law involves frequent relations with medicine, and the professional subjects most likely to arise in that connexion, together with a summary of causes célèbret, are formed into the department of medical jurisprudence. It is the practice in Great Britain to call independent medical evidence on both sides of a cause, whether the proceedings be civil or criminal.

The system of life assurance is based upon the co-operation of the medical profession. Heredity, constitution, and diathesis are here the chief subjects of general consideration, while prognosis is the skilled faculty specially called into play.

Relations of Medicine to the Body Politic.— The statutes of the United Kingdom which have direct relation to medicine are (1) those relating to the public health; (2) those relating to lunacy (and habitual drunkenness); (3) those relating to the status of the medical profession, to dentists, and to pharmaceutical chemists; (4) those relating to restrictions on the “practice” of anatomy and physiology. There are, besides, several statutes in which medicine is concerned indirectly,—such as the Poor Laws, the Prisons Acts, the Shipping Acts, the Registration of Births and Deaths Act, the Sale of Food and Drugs Act, the Sale of Poisons Act, the Factory and Workshops Act, the Artisans’ Dwellings (Metropolitan) Act, the Rivers Pollution Prevention Act, the Contagious Diseases (Animals) Act (1878), and the Public Health...