III. Regard should be had to the weather and season of the year. Dr. Hillary and Dr. Huxham both say it is much more necessary in dry, than in wet weather, and, all physicians know, it is more copiously indicated in the spring and autumn, than in summer and winter.
IV. The constitution of a patient, and more especially his habits with respect to blood-letting, should be taken into consideration, in prescribing it. If he be plethoric, and accustomed to bleeding in former indispositions, it will be more necessary, than in opposite states and habits of the system. Nature will expect it.
V. The corpulency of a patient should regulate the use of the lancet. A butcher of great observation informed me, that a fat ox did not yield more than from one half, to one third of the quantity of blood of a lean one, of the same size of bone, and it is well known, that the loss of a small quantity of blood, after cutting off the head of a fowl, is always a sign of its being fit for the table. The pressure of fat upon the blood-vessels produces the same effects in the human species that it does in those animals; of course, less blood should be drawn from fat, than from lean people, under equal circumstances of disease.
VI. As persons have more or less blood in their vessels, according to their size, less blood should be drawn, under equal circumstances, from small than large people.
VII. Regard should be had to the age of adults in prescribing bleeding. In persons between fifty and sixty years of age, for reasons formerly mentioned, more blood may be drawn than in middle life, in similar diseases. In persons beyond 70, it will be necessary to regulate the quantity to be drawn by other signs than the pulse, or the appearances of the blood, the former being generally full, and sometimes tense, and the latter often putting on the sign of the second grade of morbid action formerly described.
VIII. Regard should be had to the country or place from which persons affected with fevers have arrived, in prescribing the loss of blood. Fevers, in America, are more inflammatory than fevers, in persons of equal rank, in Great-Britain. A French physician once said, it was safer to draw a hogs-head of wine from a Frenchman's veins, than a quarter of a hundred pounds of beef from an Englishman's, meaning to convey an idea of the difference in the grades of morbid or inflammatory action in the diseases of the inhabitants of France and England, and of the difference in the quantity of blood proper to be drawn in each of them. A similar difference exists between the grades of fever in Great-Britain and America. From a want of attention to this circumstance, I saw a common pleurisy end in an abscess of the lungs, in a sea captain, in the city of London, in the year 1769, who was attended by a physician of the first reputation in England. He was bled but once. His pulse and American constitution called for the loss of 50 or 60 ounces of blood.
IX. Regard should be had to the structure and situation of the parts diseased with febrile action. The brain, from its importance to all the functions of life, the rectum, the bladder, and the trachea, when inflamed, and the intestines, when strangulated, from their being removed so much out of the influence of the great circulation, all require more copious bleeding than the same degrees of disease in the lungs, and some other parts of the body.
X. After blood-letting has been performed, the appearances of the blood should be attended to, in order to judge of the propriety of repeating it. I shall briefly describe these appearances, and arrange them in the order in which they indicate the different degrees of inflammatory diathesis, beginning with the highest.