Were it compatible with the subject of the present inquiry, it would be easy to show, that the same difficulties and evils are to be expected from Dr. Darwin's division of diseases, as they affect the organs of sensation and motion, and as they are said to be exclusively related by association and volition, that have been deprecated from their divisions and subdivisions by the nosologists. Diseases, like vices, with a few exceptions, are necessarily undisciplined and irregular. Even the genius of Dr. Darwin has not been able to compel them to move within lines. I return from this digression to remark that morbid action in the blood-vessels, whether it consist in preternatural force and frequency, or preternatural force without frequency, or frequency without force, constitutes fever. Excess in the force and frequency in the pulsations of the arteries have been considered as the characteristic marks of what is called inflammatory fever. There are, however, symptoms which indicate a much greater excess of irritating impressions upon the blood-vessels. These are preternatural slowness, intermissions, and depression in the pulse, such as occur in certain malignant fevers. But there is a grade of fever, which transcends in force that which produces inflammation. It occurs frequently in hydrophobia, dysentery, colic, and, baron Humboldt lately informed me, upon the authority of Dr. Comoto, of Vera Cruz, in the yellow fever of that city, when it proves fatal in a few hours after it attacks. In vain have physicians sought to discover, by dissections, the cause of fever in those cases, when followed by death, in the parts of the body in which it was supposed, from pain and other symptoms, to be principally seated. Those parts have frequently exhibited no marks of inflammation, nor of the least deviation from a healthy state. I have ascribed this apparent absence of disease to the serous vessels being too highly excited, and thereby too much contracted, to admit the entrance of red blood into them. I wish these remarks to be remembered by the student of medicine. They have delivered me from the influence of several errors in pathology; and they are capable, if properly extended and applied, of leading to many important deductions in the practice of physic. I shall now briefly mention the usual effects of fever, or morbid excitement in the blood-vessels, when not removed by medicine. They are, 1. Inflammation. It is produced by an effusion of red particles of blood into serous vessels, constituting what Dr. Boerhaave calls error loci. It is the second grade of fever, and, in fevers of great violence, does not take place until morbid excitement has continued for some time, or has been reduced by bleeding. 2. Secretion, or an effusion from rupture, of the serum of the blood, constituting dropsies. 3. Secretion of lymph or fibrin, forming a membrane which adheres to certain surfaces in the body. 4. Secretion of pus, also of sloughs.