If we desire to get a general idea of the changes that occur in an organ when it becomes inflamed, we must first have a knowledge of the normal structure of that organ, even though that knowledge be but superficial. Taking the intestines, for example, we see under the microscope that they are composed of layers of different tissues, called connective, epithelial, muscle, and nerve tissue; the first two forming a large part of the structure. In the connective (and fatty) tissues a great many blood-vessels are found (varying in different parts of the organ), the existence of which is necessary for the production of inflammation, since at the very outset of the process, a discharge (or exudation) takes place from these blood-vessels, accompanied by changes or degenerations in the other kinds of tissue. The process of inflammation is commonly associated with symptoms of heat, redness, swelling and pain, in greater or less degree, combined with which a change in the function of the organ is soon noticed. Micro-organisms are considered the primary cause of inflammation in many or even in most cases in which mechanical or chemical influences may undoubtedly be responsible primarily; and then again, each of these causes may be either external--that is, may originate from the outside world--or internal, that is, may be produced in and by the body itself. The first pronounced change occurring in an organ under inflammation is an increase in the rapidity with which the blood circulates through the vessels--a so-called hyperemia--which soon gives place to a diminution (stasis) in the current together with an exudation from the blood-vessels; the latter is due to changes in the structure of their walls. This exudation soon occasions a cloudiness of the connective tissues and at the same time a desquamation (shedding in scales) of the epithelia (cells of the thin mucous surface). An irritation of the nerves also takes place. The varieties of inflammation can be best apprehended by considering the different characters of the exudation. The exudation may be watery (called serous) or dense, the latter either fibrinous or albuminous. With a serous exudation there is swelling of the connective tissue and a desquamation of epithelia--the latter usually slight in character--which constitutes what is known as a catarrh; while with a fibrinous or albuminous exudation there is usually more or less destruction of the tissue itself, when, for example, we have "croup" or "diphtheria."