XI. Blood-letting should always be copious where there is danger from sudden and great congestion or inflammation, in vital parts. This danger is indicated most commonly by pain; but there may be congestion in the lungs, liver, bowels, and even in the head, without pain. In these cases, the state of the pulse should always govern the use of the lancet. XII. What quantity of blood may be taken, with safety, from a patient in an inflammatory fever? To answer this question it will be necessary to remark, 1. That, in a person of an ordinary size, there are supposed to be contained between 25 and 28 pounds of blood; and 2. That much more blood may be taken when the blood-vessels are in a state of morbid excitement and excitability, than at any other time. One of the uses of the blood is to stimulate the blood-vessels, and thereby to assist in originating and preserving animal life. In a healthy state of the vessels, the whole mass of the blood is necessary for this purpose; but in their state of morbid excitability, a much less quantity of blood than what is natural (perhaps in some cases four or five pounds) are sufficient to keep up an equal and vigorous circulation. Thus very small portions of light and sound are sufficient to excite vision and hearing in an inflamed, and highly excitable state of the eyes and ears. Thus too, a single glass of wine will often produce delirium in a fever in a man, who, when in health, is in the habit of drinking a bottle every day, without having his pulse quickened by it.