point in evidence of a constitutional morbid condition, involving a bloodchange. A striking law of symmetry characterizes certain diseases. They may be distinguished as symmetrical diseases. Examples are various cutaneous diseases, articular rheumatism, Bright's disease of the kidneys, pulmonary tuberculosis, etc. We cannot well conceive that the internal determining conditions which give rise to the local manifestations in these diseases are seated elsewhere than in the blood. We may assume it to be a rational conclusion that the essential pathology of all symmetrical diseases pertains to the blood; and the fact that the law of symmetry is exemplified in any disease, suffices to render it probable that it is a blood disease.

6. The modus operandi, so far as known, of remedies found to be useful in the treatment of a great number of local affections, shows their source to be in the blood. The remedies referred to are those which act by being absorbed into and modifying the blood. These remedies have been significantly termed alteratives. There immediate effects in the blood are unknown; we can only say that they alter in some way and affect sensibly the condition of the blood. Examples are mercury and iodine.

The foregoing are some of the considerations which establish, by logical inference, the general or constitutional origin of local affections, the nature of the general or constitutional morbid condition being unknown, but consisting, as is rationally probable, in indeterminate pathological changes in the blood. Other considerations which might be adduced are omitted, the aim being, not to enter into a discussion of this topic, but only to furnish suggestions for the inquiries and reflections of the reader.

CHAPTER VIII.

THE CAUSES OF DISEASE, OR ETIOLOGY.

Etiology a branch of General and Special Pathology-Its importance-Internal or Intrinsic, and External or Extrinsic Causes-Ordinary, and Special or Specific Causes-Traumatic Causes-Spontaneous Diseases-Primary and Secondary Causes-Complications and Intercurrent Diseases-Predisposing and Exciting Causes-Congenital and Inherited Predisposi tions to Disease-Co-operating Causes-Diathesis-Cachexia or Dyscrasia--Endemic Diseases dependent on Emanations from the Soil-Epidemic Diseases produced by Causes existing in the Atmosphere-The Causes of some Contagious Diseases exclusively derived from those affected with these Diseases, and other Diseases of this class produced, in some cases, by other Causes than Contagion or Infection-Fomites-Portability of the Special Causes of Diseases not Contagious nor Infectious.

ETIOLOGY is that branch of medicine which treats of the causes of disease. As one of the divisions of the Principles of Medicine or General Pathology, it treats of causes considered with reference, not to individual affections, but to disease in general or to different groups of diseases. The consideration of the causes of individual diseases forms an important part of special pathology or the practice of medicine. Etiology, therefore, enters into the range of topics belonging to the first and second parts of this work. Following the general plan pursued in the foregoing chapters, I shall limit myself here to certain distinctions and definitions which are to be understood as preliminary to entering on the study of individual diseases.

Knowledge of the causes of disease is highly important, in a practical

view, as the basis of prophylaxis or the prevention of disease. It is obvious that in proportion as we are able to trace diseases to their sources, we may expect to extinguish causes or obviate their morbific influence. Knowledge of causes also is important as entering into the management of diseases; for it not unfrequently happens that causes continue to be operative after disease has been produced, and their removal is, of course, a prime object in order to effect a cure. It will be apparent, when we come to inquire into the causes of individual diseases, that the amount of our present knowledge in this direction is extremely limited. It is true that modern researches have developed much information, and not a little progress has been made in the prevention and cure of diseases in consequence of increased knowledge of etiology, but there yet remains a wide field for further developments.

In treating of morbid conditions of the blood, reference has been made to certain causes of disease originating within the bodies of persons affected, and to other causes which are received from without Here is one ground of distinction. Causes are intrinsic or internal, and extrinsic or external. Examples of external or extrinsic causes are the different kinds of miasms, infectious or poisonous, the various species of virus, venoms, palpable poisons, together with the numerous agencies of disease by means of wounds or injuries. On the other hand, examples of internal or intrinsic causes are, an accumulation of urea in the blood, an excess of uric acid in gout, a morbific principle in rheumatism supposed to be lactic acid, emotional excitement, etc. Causes may be distinguished as ordinary, and special or specific. Ordinary canses are those to which all persons are more or less exposed, and which may give rise to two or more forms of disease. Atmospherical vicissitudes are ordinary causes. They are generally supposed to be involved in the causation of disease very frequently, and to give rise to a great variety of diseases Special or specific causes are extraordinary in their occurrence, that is, persons are exposed to them only at certain times or in certain situations, and each cause gives rise uniformly to one particular form of disease. The poisonous miasm which produces periodical fever, the infectious miasm which causes typhus, the virus which gives rise to smallpox, etc., are examples of special or specific causes. These causes stand in a causative relation to no other diseases than those which have just been named. The miasm which produces periodical fever is not capable of producing typhus, the miasm which produces typhus will never give rise to periodical fever, and the same is true of all other causes belonging in this category. The existence of a special or specific cause may be inferred from the special or specific character of a disease, although the nature and source of the causative agent are not understood. It is certain, for instance, that diseases possessing phenomena and laws so distinctive and uniform as those which belong to the clinical history of epidemic cholera, typhoid fever, yellow fever, etc., must each have its own special or specific cause, but the causes of these diseases have thus far eluded investigation. In fact, our knowledge of most of the special or specific causes of disease, at the present moment, does not extend beyond the fact of their existence, more or less of the circumstances connected with their development, and their manifest morbid effects in the organism.

Certain causes are distinguished as traumatic; and this name is applied also to distinguish the diseases produced by this class of causes. Traumatic canses are those which give rise to disease by acting in an appreciable manner on the part affected. Traumatic diseases are those thus produced. Anything which occasions an injury or wound of a part, and consequently disease of that part, is a traumatic cause. The name signifies a wound. A bronchitis caused by the inhalation of an irritating vapor is produced traumatically. A calculus in the pelvis of the kidney which gives rise to

pyelitis, and a stone in the bladder which occasions cystitis, are traumatic causes. The causes thus distinguished, therefore, may be either internal or external, although, in the great majority of cases, they belong to the latter class.

Diseases not traumatic, and which do not proceed from any appreciable causative agency, are said to be spontaneous. This term, as thus applied, is to be taken in a conventional, not a literal sense. It is obviously incorrect to say of any disease that it originated spontaneously. Every disease must have its adequate determining cause. But many diseases are developed without our being able, in the existing state of knowledge, to refer them to their causes; and the term spontaneous, applied to those diseases, means simply that they proceed from causes which our present knowledge does not enable us to appreciate. Pleuritis, pericarditis, and other affections developed frequently in the course of Bright's disease, were formerly considered as spontaneous; but now that their development is attributable to the action of urea accumulating in the blood, the term has ceased to be applicable to them. The number of so-called spontaneous diseases will diminish in proportion as our knowledge of etiology advances.

Causes are distinguished as primary and secondary. A secondary cause is an effect of disease. Urea, for example, accumulating in the blood, gives rise to various local affections-pleuritis, pericarditis, etc. But the accumu lation of urea is an effect of some disease of the kidneys, which interrupts the eliminative function of these organs. Urea in excess in the blood, is thus a secondary cause of disease. A primary cause, on the other hand, does not proceed from a prior disease. All external traumatic causes, for instance, are primary. The same distinction is made among diseases; that is, some are primary, and others secondary. Primary diseases proceed from primary causes, and those called secondary are due to the effects of pre-existing diseases; in other words, to secondary causes. A large proportion of the diseases, the existence of which is first ascertained, are secondary; they involve in their causation antecedent affections. Not unfrequently a number of diseases are thus linked together in a consecutive series. In the examination of patients, therefore, it is not sufficient to have discovered a disease; but, having discovered one disease, it remains to be ascertained whether one or more additional diseases do not exist, sustaining to the disease first discovered a relation either of cause or effect. Clinical researches have developed important facts with respect to the causative relations existing between different diseases, and these facts are of great utility in medical practice.

Secondary and subordinate affections are distinguished as complications. A complicating disease is one which occurs in the course of another disease with which it is connected by some pathological relation. Pneumonitis, for example, developed in the course of typhoid or typhus fever, is a complication of the latter. It is consecutive to the fever, and to a greater or less extent dependent thereon for its production. Pneumonitis generally has a complication, viz., circumscribed pleurisy. A disease is said to be intercurrent when it occurs during the progress of another disease, without necessarily having any dependence on the latter. Lobar pneumonitis, for example, occurring in a patient affected with pulmonary tuberculosis, is an intercurrent affection.

As just stated, certain causes of disease are themselves effects of disease; so that certain diseases may be said to stand in a causative relation to other diseases. On the other hand, some diseases afford protection against others.

'Protopathic and deuteropathic.

As an illustration, emphysema of the lungs and pulmonary tuberculosis may be cited. The latter disease occurs in patients affected with the former so rarely as to show that it exerts a protective influence. Another illustration is afforded by disease of the heart and tuberculosis of the lungs. A tuberealous deposit occurs so rarely, when cardiac disease occasions much disturbance of the circulation, that the latter must be considered as affording more or less exemption from the former.

Another division is into predisposing and exciting causes. Predisposing causes induce a liability or tendency to certain forms of disease. Exciting causes, acting on persons already predisposed to certain diseases, determine their occurrence. Predisposing causes may be alone sufficient to give rise to disease, or they may only suffice to place the system in a condition favorable for the occurrence of disease; in the latter case the disease occurs when exciting, are superadded to the predisposing, causes. To cite an instance: A person under the influence of predisposing causes is in a condition favorable for the occurrence of acute rheumatism; in this condition he is exposed to atmospherical vicissitudes, and the latter determine an attack of this disease. The exciting cause, viz., the atmospherical vicissitudes, would not, alone, have been adequate to give rise to an attack of rheumatism; and the former, viz., the predisposing cause, alone, might have been inadequate; but, conjointly, they occasion this disease. Exciting causes act the part of the match when everything is ready for an explosion. It is thus seen that predisposing causes determine the nature of the disease, and exciting causes the date of its occurrence.

A predisposition to some form of disease may be inherent in the constitution, or congenital. Some persons have inborn tendencies to certain diseases. This fact is strikingly illustrated by the occurrence of phthisis successively in different members of the same family-a large number of brothers and sisters being carried off by this disease. This congenital predisposition may remain completely latent until the period of life in which the disease is most apt to be developed; and we sometimes see a whole family of children, one after the other, fall victims to this disease, when they severally reach a certain age. A predisposition to certain forms of disease may not only be congenital, but inherited; a constitution involving a tendency to disease is transmitted from parent to offspring. Among the diseases perpetuated in this way are phthisis, gout, cancer, asthma.

The predisposition to a disease, whether congenital or acquired, may be strong or feeble. Various circumstances may act as co-operating causes; that is, acting in furtherance of the constitutional tendency. Thus, in a person predisposed to phthisis, the development of this disease may be greatly promoted by unfavorable hygienic circumstances, such as sedentary habits, deficient ventilation, and inadequate alimentation. These co-operating causes, taken in conjunction with an existing predisposition, may serve to develop the disease, when, without their aid, the predisposition might not have been sufficient. There is reason to believe that persons with a feeble predisposition to this disease often escape, if they are exempt from the operation of co-operating causes. The latter, moreover, are to a greater or less extent controllable, while the predisposition, especially if it be congenital, is entirely beyond control. Diseases thus are preventable, notwithstanding a predisposition to them, Tn so far as they depend on the union of co-operating causes. Herein lies a truth of great practical importance.

Diseases which originate from special causes often appear to require for their production co-operating causes. Facts seem to show, for examples, that yellow fever, periodical fevers, and epidemic cholera, which undoubtedly involve the agency of special causes, would in many cases not have occurred,

had not other than the special causes contributed to their occurrence. By eradicating, as far as possible, all unfavorable hygienic influences, the special causes of these diseases may be rendered, to a great extent, or perhaps completely, inoperative. The special causes of disease and the co-operating causes are, to quote the comparison of the late Dr. Barton, like the two blades of a pair of scissors-conjoined they are effective, but disunited they effect nothing. The special causes of these diseases we may not be able to remove; but co-operating causes are, to a great extent, within our control; and, by removing the latter, the diseases are rendered preventable.

A constitutional predisposition to a particular form of disease constitutes what is called a diathesis. A diathesis, therefore, may be either congenital or acquired. And the diseases which are considered as generally, if not always, involving a constitutional predisposition, or diathesis, are sometimes distinguished as diathetic diseases. A cachexia or dyscrasia involves a diathesis, and something more. These terms denote not merely a constitutional predisposition to a disease, but that condition of the system which exists when the disease is actually developed. A person born with a tendency to phthisis, for example, has the tuberculous diathesis; and this diathesis eventuates in the tuberculous cachexia when the person becomes affected with phthisis.

Our knowledge of the special causes of disease, as already stated, is extremely imperfect as regards their nature, source, and the modus operandi by which they give rise to morbid manifestations But certain conclusions respecting their origin and diffusion may be logically determined. One of these conclusions is, that some poisonous miasms emanate from the soil. This may be inferred with respect to the special causes giving rise to diseases called endemic. Endemic diseases are those which prevail within circumscribed territorial limits; that is, their prevalence does not extend beyond sectional boundaries. Now, this fact is sufficient for the inference that the source of the causative miasms is in the soil; because, of the elements which enter into climatic influences, those peculiar to any particular district are terrestrial. Again, in the diseases called epidemic, viz., those which prevail successively or simultaneously in different and often wide-spread territorial districts, the poisonous miasms, whatever may be their source, must be diffused through the atmosphere. The causes of epidemic diseases are migra tory. In some instances they traverse successively almost every portion of the habitable globe. This is true of epidemic bronchitis or influenza, and of epidemic cholera. It is altogether improbable that the special causes in these and other epidemics are developed in the different sections of country over which their prevalence extends. They are produced in the situation whence they take their point of departure, and travel, in some way, through the atmosphere. The special causes which give rise to contagious and infectious diseases are derived from the bodies of those affected with these diseases; and, with respect to certain of these diseases, it is probable that the special causes are exclusively thus derived. It may fairly be doubted whether smallpox ever originates from any other source, notwithstanding the occurrence of cases in which it cannot be traced to contagion or infection. Yet, the first case of smallpox which ever existed must, of course, have been an exception to this law, and it is possible that there are still occasional exceptions. With respect to certain of the diseases which are communicable, it is probable that they not unfrequently originate otherwise than by contagion or infection. This is true of typhoid, and perhaps also of typhus fever.

Diseases are communicated, not only by immediate contact, and by an infectious miasm diffused in the atmosphere, but by means of what are called