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.

CHAPTER IX.

Of the Treatment of Fever.

WE have seen that the first indication of disease in fever is traceable to the nervous system; that the nature of this primary affection of the nervous system is unknown; that it may possibly be the commencement of inflammation, modified by the nature of the nervous substance, in which the inflammatory action has its seat, and by the nature of the cause that excites it, namely, a peculiar poison : or, on the other hand, it may possibly be something distinct from inflammation, but having a peculiar tendency to excite it. In either case, the inflammation that is present in fever, is peculiar and specific, differing essentially from ordinary or simple inflammation. Whether the affection of the nervous system consist merely of inflammation of the nervous substance excited by a peculiar poison; or whether it consist of some unknown condition of the nervous system to which inflammation is superadded, and by which the character of that inflammation is modified, the great practical result is the same,

namely, that febrile inflammation and ordinary inflamination are not identical, and that the difference between the two affections is such as to require a very considerable modification in the treatment appropriate to each.

The only morbid condition of fever, of which we have any knowledge, and over which the medical art has any control is that of inflammation. Although, as has been so often stated, inflammation be not the primary febrile affection, as far as regards the order of events, yet it is, at least, the primary affection, as far as regards the treatment, if it be not the sole affection that admits of treatment. The remedies proper for febrile inflammation do not differ from those which are adapted to ordinary inflammation; but they differ materially in the mode in which they ought to be applied, and the extent to which they onght to be carried. They can be understood neither in their mode nor measure, until the following qucstions are determined; namely, What is the precise object that should be aimed at in the treatment of fever? What is it which it is most important to do, and which it is in the power of the medical art to accomplish? An exact and true answer to these questions will afford an invaluable guide in practice: it will point out with clearness what is to be attempted; and it will put a stop to useless and pernicious aims.