The nature of a disease being determined by the condition of the individual and the exciting cause to which he is exposed, the next division of pathology is the study of the symptoms or signs by which the progress of a disease is marked, and by which in practice its nature is to he determined. Of these signs of disease, many are expressive of the altered condition of the part chiefly affected, as pain in a wound, or a local inflammation, coughing in a disease of the lungs, sickness in a disorder of the stomach : but a greater number are the expressions of an affection of other organs, which stiffer in association with those primarily diseased ; such are pain in the head when the digestion is disordered, coughing in diseases of the liver, sickness after violent blows on the head. Sometimes these secondary symptoms completely meek those immediately resulting from the primary disease ; as when in a disease of the hip the chief pain is felt in the knee, or in hysteria any organ may appear disordered except that which actually is. These secondary symptoms are ascribed to what is called sympathy, an unsatisfactory expression indicating only the coincidence and pro bable connection of symptoms of disease in two organs, of which one only is supposed to be materially affected. Entire ignorance must be confessed of the nature and origin of many of these sympathetic or indirect symptoms of disease ; as, for example, of the fever consequent on local injuries or acute local diseases, and of the hectic fever of many chronic affections ; but it is probable that all sympathies will in time be found to depend either on some communication of excitement from one nerve to another through the medium of the spinal chord or brain, as in the reflex actions [NERvous SYSTEM, in NAT. HIST. Div.], or on some change in the blood which affects both organs at once, or which, originating in the disease of one disturbs the functions of the other, or of the whole body.
Whether directly or indirectly produced, all the symptoms of disease are only the perversions of the natural functions of the part affected, or appreciable changes in its structure ; their value and meaning there fore can only be determined by a comparison with the same functions and structure in health ; in other words, this, like all other parts of pathology, cannot be rightly studied without a constant reference to physiology. It is believed that an organ may be only functionally deranged ; that is, that its several functions may be performed in a very unhealthy manner, without tho existence of any material change in its structure and composition. These are called functional symptoms, but their number is probably much less than is generally believed, and it is most likely that they are limited to the variations to which the organs are subject by the changes in the mode and measure of the influence of the nerves upon them. For all other symptoms we mast assume the existence of a substantial change in the part affected, or in the materials on which it has to act, although in many cases these changes are fugacious or inappreciable by our senses.
For a due performance of all the functions of organio life[VITA LITY, in NAT. Hist. Div.], a healthy structure of each organ, and a healthy composition of the blood, on which they all act and all de pend for their own maintenance, are alike necessary; a deviation from health in either will therefore produce the symptoms of disease ; a conclusion in which the long continued disputes of the humeral pathologists, who ascribed all disease to the blood, and the eolidista, who held all to depend on changes of structure, have at length merged. To these two kinds of alterations, and to perturbations in the distribution of the nervous influence, it is probable that the signs of all diseases may be referred • but from the peculiar and complicated nature of the animal and the universal connec tion between all its organs, no one of these changes can long con tinue without producing the others; and hence in diseases of any degree of seventy the symptoms are commonly a mixture of the dis orders of all the functions of the body, and the disorder of each is modified by the changes in all those circumstances on which its healthy state depended, as the condition of the blood, of the nervous influence, &e. Neither are the symptoms in any case constant pheno
mena; for the Influences of all external circumstances upon a dis eased body are very different from those which they exert upon the healthy body, and many things which were necessary to health are supporters of disease, as the usual amount of food, of bodily and mental exertion, so that exclusion from them becomes neces sary, and this again further modifies the performance of the disordered functions.
The history of a disease is completed by the process of natural recovery or by the observation of the changes in the structure of the body which it produces. The influence of remedies cannot justly be considered as a branch of pathology ; though most important for their utility, still, in their relations to the natural history of a disease, medicines can only be regarded as interfering circumstances, or as the means of experiments for the determination of the relation of the dis eased body to particular agents, by which the nature of the disorder affecting it may be sometimes ascertained.
The recovery from disease is an example of the exercise of that power by which the body can make unusual efforts to prevent its own destruction : this has been called the via medicatrix naturx, or curative force of nature. It is exerted in many cases in which 'dis ease cannot be said to exist, but where rather there is an exaggeration of ,health; as, for example, when a muscle subjected to unusual exer tion and an unusually great amount of waste not only repairs its loss, but actually becomes larger and stronger, so that it can bear the same amount of constant waste better than at first ; or as when a person is exposed for a time to cold in bathing, the speedy consequence is an increased warmth of the surface. The term reaction is applied to phenomena of this kind, and it may be said that reaction takes place whenever any injurious influence is applied to the body. In eimple cases the reaction effects at once a restoration to health, as in the instances above mentioned ; in others the reaction is itself the most prominent feature of the disease, as in fever and inflammation.
The recovery from disease ia rarely perfect. Although no visible change may be left behind, yet the part diseased is commonly for ever after weak, that is, more than usually liable to the same or to some other disease. It is probable that this liability is owing to some morbid change in the structure of the part inappreciable by our present means of examination ; in more distinct cases, when any part has been severely diseased, we never see a perfect restoration of its healthy structure and forin. Even in those tissues that are most easily re paired, there is not an actual reproduction of the injured structure.
The period occupied in the progress of a disease to recovery or death is the basis of the chief division of acute and chronic diseases. The severity of the symptoms may in both cases be the same ; but in general those of chronic cases are less prominent than those of acute oases.
When the disease terminates fatally, or when death takes place from any other cause at a distant period from its occurrence, we obtain perhaps the most valuable because the most certain part of pathological knowledge, that of the material effects which the morbid process has produced. This, the study of morbid anatomy, is often specially called pathology. By the examination of the altered parts and a comparison of the changes of structure which they present with those which are known by observation of external diseases or by experiment to result from certain leading morbid processes, as inflam mation, &c., we are enabled to determine the nature of that which had existed beyond the limit of our senses, and thus to appreciate correctly the meaning of the several symptoms which had marked its progress „during life, and the powers and mates of action of the circumstances to which it owes it origin. The practical value of such knowledge is the power which it affords of determining during life the nature of each disease, and the appropriate remedy for each.