The two terms, sign and symptom, are constantly used to express the same idea. There is, however, in reality a well-marked difference between them. A symptom is one of the characters of a disease, just as a cough is a symptom of an ordinary cold; it is also a symptom of acute bronchitis and laryngitis, as well as a symptom of what is known as broken wind; while a sign is a definite indication of a particular disease, as the presence of the tubercle bacillus is a sign of the existence of tuberculosis.

Notwithstanding the admitted difference between the signification of the two terms, they always have been, and probably always will be, used interchangeably. Strictly speaking, however, this is only justifiable when symptoms are diagnostic, in which case the term signs may be properly applied to them.

Symptoms may be described as local and general, according to whether they are limited to the diseased part or relate to the whole of the organism. They are also described as idiopathic when they arise directly from the diseased part, and sympathetic or secondary when they are due to secondary disorder. They are also premonitory and precursory when they are of a nature to suggest the advent of a disease, the indications of which are not yet defined. Thus it may occur to an attendant or to the owner of a horse that the animal has something the matter with it, but the most careful inspection and enquiry may fail to lead to the discovery of any precise morbid condition. Neither the pulse nor the breathing exhibits any special characters, and all that can be gathered from the animal's condition is the impression that it is sickening for something; and if it should be the case that influenza is prevalent in the district, the suspicion is at once aroused of the infection having attacked the animal.

Symptoms are diagnostic when they indicate the precise nature of the disorder from which the animal is suffering; for example, an attack of violent but intermittent abdominal pain is a diagnostic symptom of spasmodic colic. Prognostic symptoms include all those indications of disease which suggest the probable termination, as a failing pulse and coldness of the surface are warnings of a fatal ending to the disorder. Therapeutic symptoms are those which indicate a particular line of treatment, as high temperature, quick pulse, and rapid breathing show the existence of fever, and point to the application of febrifuge remedies.

Symptoms which are obvious to an observer are described as objective, but when they are only expressed or described as sensations experienced by the patient they are called subjective symptoms. It is evident that in the lower animals subjective symptoms are practically non-existent, as it is rarely the case that an animal by its actions can express its sensations in such a manner as to be rightly interpreted. Again, symptoms are called dynamical when they are active or violent, and statical when they are subdued; the terms positive and negative are also used to express the same conditions.

Pathognomonic is a term used in application to symptoms which indicate a particular disease, as a peculiar cough and double action of the expiratory muscles are pathognomonic of broken wind. To the unprofessional reader these terms may appear to be unnecessarily complicated, but a little consideration will prove their usefulness as a means of saving time in description.