It will be evident that too much reliance must not be placed upon a single observation, particularly in emergency cases. Whenever possible, a series of observations should be made, the blood being examined about four hours after meals, and about the same hour each day. The clinical significance of the blood count in individual diseases will be further referred to. _The Iodine or Glycogen Reaction._--The leucocyte count may be supplemented by staining films of the blood with a watery solution of iodine and potassium iodide. In all advancing purulent conditions, in septic poisonings, in pneumonia, and in cancerous growths associated with ulceration, a certain number of the polynuclear leucocytes are stained a brown or reddish-brown colour, due to the action of the iodine on some substance in the cells of the nature of glycogen. This reaction is absent in serous effusions, in unmixed tuberculous infections, in uncomplicated typhoid fever, and in the early stages of cancerous growths. CHAPTER III INFLAMMATION Definition--Nature of inflammation from surgical point of view--Sequence of changes in bacterial inflammation--Clinical aspects of inflammation--General principles of treatment--Chronic inflammation. Inflammation may be defined as the series of vital changes that occurs in the tissues in response to irritation. These changes represent the reaction of the tissue elements to the irritant, and constitute the attempt made by nature to arrest or to limit its injurious effects, and to repair the damage done by it. The phenomena which characterise the inflammatory reaction can be induced by any form of irritation--such, for example, as mechanical injury, the application of heat or of chemical substances, or the action of pathogenic bacteria and their toxins--and they are essentially similar in kind whatever the irritant may be. The extent to which the process may go, however, and its effects on the part implicated and on the system as a whole, vary with different irritants and with the intensity and duration of their action. A mechanical, a thermal, or a chemical irritant, acting alone, induces a degree of reaction directly proportionate to its physical properties, and so long as it does not completely destroy the vitality of the part involved, the changes in the tissues are chiefly directed towards repairing the damage done to the part, and the inflammatory reaction is not only compatible with the occurrence of ideal repair, but may be looked upon as an integral step in the reparative process.