This subject, it is my intention to take up, and to discuss fully in a future publication, in which will also be investigated some inquiries, which it has been found impossible to include in the present volume; such as whether the vegetable and animal poison we have been considering, be the only true exciting cause of fever; by what means its general diffusion is effected; on what conditions its propagation depends; by what measures its extension may be checked, and its power diminished or destroyed; what circumstances in the modes of life, in the habits of society, in the structure of houses, in the condition of the public streets and the common sewers, in the state of the soil over large districts of the country, as influenced by the mode of agriculture, drainage, and so on, favour or check the origin and propagation of this great curse of civilized, no less than of uncivilized man. It is obvious that these inquiries will include the investigation of several exceedingly curious and important statistical questions; and the object of these researches will be accomplished should they lead to the establishment of any useful principles of extensive application.[[34]]
II. Of the Remote or Predisposing Causes of Fever.
The remote or the predisposing causes of fever have been stated to be those circumstances which bring the body into a condition capable of being affected by the immediate or the exciting cause. Whatever diminishes the vigorous action of the organs, impairs their functions, and so weakens the general strength of the system, is capable of becoming a predisposing cause of fever; and every predisposing cause acts in one or other of these modes, and becomes a predisposing cause only and in proportion as it lessens the energy of the system, or disturbs the balance of its actions, which in fact is to render some portion of it weak. During a state of vigorous health the body is endowed with the power of resisting the influence of noxious agents, which in a less perfect state of health are capable of producing intense and fatal disease; and the action of all predisposing causes is to lessen this resisting power, or to weaken the energies of life.
Of all predisposing causes, the most powerful is the continued presence and the slow operation of the immediate or the exciting cause. It is a matter of constant observation, that the febrile poison may be present in sufficient intensity to affect the health, without being sufficiently potent to produce fever. In this case the energy of the action of the organs is diminished, their functions are languidly performed, the entire system is weakened, and this increases, until at length the power of resistance is less than the power of the poison. Whenever this happens, fever is induced; not that the power of the poison may be at all increased; but the condition of the system is changed, in consequence of which, it is capable of offering to the noxious agent that assails it less resistance.
We have seen that the vegetable or animal poison may exist in sufficient intensity to produce fever on the slightest exposure to it, without the operation of any predisposing cause, in a body in the state of the soundest health, and endowed with the greatest degree of strength. Examples of this kind are but too frequent in tropical climates. In countries where the temperature never rises so high, and seldom continues so long, it is rare that fever is produced immediately, on exposure to the exciting cause. Concentrated and potent as that poison is in many parts of Flanders, yet Sir John Pringle states that, in removing to an unhealthy situation, the men rarely became ill at once; that they generally continued in tolerable health for some days; and that recruits recently arrived in the country, resisted the noxious agent longer than the men who had been long there. Dr. Potter gives a remarkable example of the same fact, with regard to the yellow fever, which fell under his own observation, and states other facts, strikingly illustrative of the influence and operation of the predisposing causes. Strangers, from certain countries, he informs us, are insusceptible of yellow fever in America. In the most malignant and protracted epidemics which afflict that country, these strangers uniformly escape: emigrants from the West Indies, and other warm latitudes, for example, invariably resist the cause which produces these maladies in the native inhabitants. But the curious fact is, that such persons are unable permanently to resist the operation of the exciting cause; for, after a residence in America of some years, their constitution is so completely assimilated by the influence of the climate to that of the American, that they become equally sensible to its febrile miasma, and are as exquisitely impressed by them, as the American citizens themselves. The illustration is equally striking and instructive, if the position be reversed. The natives of northern climates are extremely susceptible to the influence of these miasma; that susceptibility is in exact proportion to the latitude of their country: those from the north of Europe scarcely ever escape an attack; the natives of Great Britain are nearly as susceptible to the influence of the poison, while persons even from the more northern countries of the United States are more liable to the disease than the citizens of the southern and middle states.
Dr. Potter performed some experiments, to show that the continual presence of the exciting cause not only operates upon the general system, but actually produces a morbid change in the blood, before it induces fever. During the prevalence of an epidemic, it was observed that, in all the cases in which the patients were bled, the general appearance of the blood was precisely the same; that the coagulum was either of a yellow, or of a deep orange colour, and that a portion of the red particles was invariably precipitated. It occurred to Dr. Potter that, if the cause of the disease were contained in the common atmosphere, the blood of those who had inhaled it a certain time would exhibit similar phenomena; and that, should this be the case, it would prove that the cause, before actually producing the disease, brought about a state of the system, which predisposed it to be affected by the poison. To ascertain the appearances of the blood in persons who were exposed to the febrile poison, but who still remained apparently in perfect health, he drew a quantity of blood from five persons, who had lived during the whole epidemic season in the most infected parts of the city. To external appearance and inward feeling, each of these persons was in sound health. Their blood could in no respect be distinguished from the blood of those who laboured under the most intense forms of the prevailing fever. As it was necessary to the conclusiveness of the experiment that their blood should be compared with the blood of those who lived in an atmosphere unquestionably pure, Dr. Potter selected an equal number of persons who dwelt on the hills in Baltimore country, and drew from each of them ten ounces of blood. The contrast was most manifest. The serum was neither of a yellow, nor of an orange colour; there was no red precipitate; the appearances were such as are found in the blood of persons in perfect health.
A young gentleman having returned to the city from the western part of Pennsylvania, on the 10th of September, in a state of sound health, Dr. Potter drew a few ounces of blood from a vein, on the day of his arrival; it exhibited no deviation from that of a healthy person. He remained in the family until the 26th of the month, that is sixteen days. On the 16th day the bleeding was repeated. The serum had assumed a deep yellow hue, and a copious precipitation of red globules had likewise fallen to the bottom of the vessel.
In these experiments, the blood in six persons indicated the operation of the morbid cause, while each remained in a state of apparent health. Of these six persons, four were actually seized with yellow fever during the prevalence of the epidemic; and the other two, though they escaped any formal attack, did not escape indisposition. They were affected with headache, nausea, and other indications of disease, like hundreds besides, who were never absolutely confined to the house, and who never took any medicine, but who still experienced in nausea, giddiness, headache, pain in the extremities, and so on, abundant intimations of the presence of the poison.
These examples may suffice to show how the exciting, may itself become a most powerful predisposing cause. The predisposition to subsequent attacks, after the system has once suffered from the disease, is very remarkable; that predisposition remains for a considerable period after convalescence and apparent recovery. Of this, striking examples continually occur both with regard to intermittent, and to continued fever. In fact, the disposition to relapse, remains until the constitution has recovered its previous strength and vigour, however distant that period may be. The influence of cold, moisture, fatigue, intemperance, constipation, anxiety, fear, and all the depressing passions, are likewise extremely powerful predisposing causes. They enable a less dose of the poison to produce fever, and they increase the intensity of the fever when it is established. They all act by weakening the resisting power inherent in the constitution, that is, by enfeebling the powers of life.