Introduction

PATHOLOGY is the scientific study of disease. The term Disease, which strictly means nothing more than Discomfort, has reference primarily to the subjective sensations of the person affected, but as these feelings have an objective basis, and as it is possible to distinguish various groups of phenomena, each with its own mode of divergence from the healthy conditions, these groups of phenomena have come to be designated as diseases.

In Pathology we aim at getting close to the various diseases, so as to study their essential nature. It is a scientific study, and it is pursued by scientific methods. There are three principal methods by which the nature of disease is elucidated, as there are three main sources of our knowledge of disease. These may be designated respectively, Experimental Pathology, Pathological Anatomy and Histology, and Clinical Observation. To each of these is Pathology indebted in endeavouring to form a conception of each disease in its true characters.

So far as it can be prosecuted, Experimental Pathology, which aims at the production of morbid conditions in animals so as to observe them more closely, is calculated to afford us the most direct and trustworthy information regarding the nature of diseases, and much of the advance in modern pathology is due to this line of study. If we can actually induce a given disease in an animal, we already know a great deal in regard to its causation, and are in the way of finding out much as to its nature. In some cases we are able actually to observe the morbid processes, by means of the microscope or otherwise, in the living animal; in others we can kill the animal at various periods after the onset of the disease, and observe the results at various stages. We can vary the experiment in different ways so as to eliminate on one side and another the various elements, and so reduce the problem to its simple and necessary parts. Experimental pathology has thus carried us far towards understanding many of the conditions in disease. Reference need only be made to the reformations which this line of study has effected in our knowledge of Inflammation, Embolism, Tuberculosis.

The study of Morbid Anatomy may not effect such revolutions in pathology as experimental pathology, but it is by this method that the scientific basis of pathology has been laid, and, in the training of the individual medical student, it is calculated to play a more important part in giving him an understanding of the nature of the diseases with which his professional work will afterwards bring him in contact. Morbid anatomy deals with the physical changes which disease produces in the body. These changes are, as it were, the expression of the pathological processes, and are only to be regarded as indications of the existence of these processes. It is probable that all morbid processes produce changes in structure, but the changes may sometimes elude our observation. For the most part, however, we are able to associate the change of structure with the morbid process, and the observed change leads us up to the actual nature of the disease.

Morbid Anatomy is the study of the structural changes in all periods and stages of the disease. It is sometimes objected to this mode of study, that in observing the state of the tissues after death we see only the last stages of disease, and in some cases only the results. This, however, is not the case. Every clinical observer will admit that in the same disease the fatal issue occurs, in different cases, under the most diverse circumstances. Death ensues, as a rule, when by reason of defective nutrition, or a deficiency in the supply of oxygenated blood, the essential functions of the body can no longer be carried on. As there is in different individuals an infinite variety in the vigour of the vital processes, so a condition which in one will very slightly affect the essential phenomena of life, will, in another, seriously compromise them. This is an experience of our every-day life. Hence it may be said in any given disease, the appearances after death will, in different cases, show diverse stages and phases of the disease. Indeed, even a single case may show, in different parts of the affected organs, the various changes in their different stages.

As an illustration of what is meant, take the case of pneumonia or acute inflammation of the lungs. Patients die in this disease, as a rule, from failure of the heart and consequent cessation of the circulation; but the paralysis of the heart is not related mainly to the condition of the lungs. It is more closely related to the elevation of temperature and alteration of the blood which are the consequences of the absorption of poisonous products from the lungs, while the power of resistance of the heart in each person is an important element in the matter. And so it happens that one man will die with a limited and advancing pneumonia, while another will survive a very extensive involvement of the lung. The lung at death in the one case is in the same stage of the disease as in the other at a certain date during life. We can study the nature of the disease in the former and we are surely warranted in assuming that, in the patient who survived, the state of the lung is virtually identical with that in the patient who died.

Again, in a carcinoma growing up to death, we have, at the growing margins of the tumour, changes which are the same as those by which the tumour has all along been extending, and we can study the methods of its growth as well as its relations to the structures in which it has been growing.

Thus it is that, although morbid anatomy by itself may in some cases throw little light on the nature of the disease, yet, taken as an expression of the morbid process, it is of the highest value.

In referring to processes and changes it must be understood that the finer structure of the body is taken into account. In Pathological Anatomy, we include Pathological Histology, and it may indeed be said that, as the coarser, naked-eye appearances are merely the expression of the finer structural details, so we shall not under stand the former unless we are able to refer them to the latter. We can have no accurate idea of the proper structure of the normal liver by simply looking at it with the naked eye, and we can have little insight into its morbid conditions by the same method of observation. The morbid conditions, however, do generally cause changes in the naked-eye appearances, and this is of great consequence as enabling us to infer, even from the naked-eye appearances, the character of the change in structure. But these macroscopic appearances are only of value when they are used to throw light on the microscopic changes or to direct attention to them. The really important processes are those which concern the finer elements of the tissues, and so it is to these that we shall chiefly attend in studying the various forms of disease.

Clinical Observation is of great importance in the study of pathology, and that in various ways. It is the processes occurring during life that are the real subject of study in pathology, and it is only by careful clinical observation that we are able to get, as it were, into close relations with these processes during their actual currency. Clinical Observation and Pathological Anatomy should thus be taken as mutually elucidating each other, and as working together towards just conceptions as to the nature of the conditions concerned. *

There is another aspect in which clinical observation is of importance in relation to pathology. The pathologist, if conversant only with morbid anatomy and experimental pathology, is apt to form a too mechanical and simple conception of the processes involved, and is likely to theorize on the basis of a too limited view of the subject. Clinical Observation is a needed corrective to this, bringing the pathologist back to the practical aspects of the matter, and checking the tendency to theorize by the discipline of facts.

In the systematic treatment of the various forms of disease, the plan here adopted has been to take up first the diseases in their general aspects, and afterwards the diseases of the special organs and systems. This work, therefore, divides itself naturally into a General Part and a Special Part.

In the General Part will be taken up those diseases which are not confined to any organ or tissue: most of them, indeed, may affect any organ of the body. This is preceded by a Section on the Nature, Causation, and Terminations of Disease.

In the Special Part, the diseases as they manifest themselves in the individual organs and tissues are considered. It is clear that the diseases described in the General Part will again come under review in each division of the Special Part, and they will be taken up very much in the same order as in the General Part. But each disease presents various specialities according to its locality, and, from a practical point of view, it is important to understand the modifications thus occurring. The Special Part will be, in a certain sense, an expansion and amplified illustration of the general principles educed in the General Part.