The transition of a mild case of fever into a severe one, or the progress of a case severe from the commencement, is accompanied with, or depends upon, as will abundantly appear hereafter, certain changes that take place in certain organs. These changes occur with great regularity; the organs in which they take place are always the same; and the symptoms by which they are denoted are uniform. The organs affected are the spinal cord, the brain, the membranes of both, the mucous membrane of the lungs, and the mucous membrane of the intestines. For the reason just assigned the nature of these affections cannot be described in this place, but must be postponed to that part of the work which treats of the pathology. Since however the symptoms are nothing but the signs of these conditions, and the history of the succession of the former, is nothing but an account of the indications of the successive changes that take place in the latter, all the important symptoms must necessarily have their seat in the head, in the thorax, and in the abdomen. Mixed and blended as they appear in the different cases which the practitioner is called upon to treat, nothing can appear more complex or more variable: when analyzed, nothing is more remarkable than their simplicity and their uniformity. In order to perform that analysis with exactness, and to render it really instructive, these symptoms must be contemplated as they arise in the affected organs. These organs, as we have seen, are the cerebral, the thoracic, and the abdominal; the symptoms therefore divide themselves into cerebral, thoracic, and abdominal: there is, indeed, a fourth order, in which all the organs appear to be equally involved; in which the general affection is intense, and which therefore may be appropriately termed mixed. We shall see that cases of this kind constitute by far the most dangerous form of the disease. I. SYNOCHUS GRAVIOR WITH CEREBRAL AFFECTION, occurs under two degrees of intensity: when the cerebral affection is moderate, it may be termed subacute; when great, acute.