As we have elsewhere pointed out, joints vary very considerably, not only in their structure, but also in the purpose they serve. Some, as those by which the bones of the head are united, are fixed and immovable, while those of the extremities enjoy a considerable range of action, and upon their liberty or freedom of movement depends the power of locomotion. The former are formed solely by two bones held together by the interlocking of their serrated edges, while the latter comprise not only two or more bones whose articular surfaces are encrusted with cartilage, but these are enclosed in a synovial membrane spread over the internal surface of a sac termed the capsular ligament. In addition, strong bands of fibrous tissue, in the form of "connecting ligaments", unite the bones together. This form of joint is known as the diarthrotic, and is the most free-moving of the several varieties.

A third variety of joint is found uniting the bodies of the vertebrae. Here, between the bones, a dense cushion of fibrous tissue is interposed. It has no synovial or capsular membrane, as in the case of the diarthrotic variety of joint, and its range of movement is very restricted, being only so much as to impart to the spine a limited degree of flexibility and strength. This is known as an amphiarthrotic joint.

Of the three varieties of joints above referred to, it is very seldom that disease is found to exist in any but those of the diarthrotic class, and in this section the joints of the extremities afford by far the greater number of examples. Their larger size, more exposed position, great range of action, and liability to jar, concussion, and sprain, are no doubt the factors which tell in favour of disease.

Joint diseases are brought into existence either by local or constitutional conditions, or both, as when the latter predispose to the influence of the former.

If there is one period of life when horses suffer more from joint disease than another, it is from birth up to six years old, when the burdens they have to bear are badly apportioned to their condition and strength, and when certain constitutional ailments which determine these affections are most prevalent.

External violence is perhaps the most common exciting cause of joint disease. Kicks from other horses, slips, falls, blows, sprains, and penetrating wounds by which inflammation more or less severe is excited in one or another of the component structures, are the chief inducing factors. Fractures, by extending into the joints, not only provoke in them acute and dangerous disease, but so far damage and impair their action as seriously to compromise further usefulness. Wounds in connection with joints are of especial difficulty and danger, not only on account of the joint having been opened, but because of the entrance into it of septic organisms which excite a suppurating or matter-forming process, first in one structure and then in another, until all have become implicated.

Of those constitutional conditions to which joint disease can be referred, rheumatism offers a striking and by no means an uncommon example.

Within sixty yards of where the writer now sits is a gray cob which, while passing through an attack of influenza fever, suddenly became acutely lame in both fore-limbs, so much so that her movements could only be compared to those of an animal in the last stages of chronic navicular disease. Every joint and sinew in the limbs was perfectly normal, but the heat in the feet and the fulness in the heel clearly indicated some mischief in the navicular joint, which was diagnosed as rheumatism. A blister was applied over the coronets and the mare was turned to pasture, when she became perfectly sound in three weeks. Rheumatism affecting the joints and other structures presents itself also as a sequel of strangles and purpura.

In that pyaemic condition of foals known as "navel ill", the large joints of the extremities frequently become distended with pus and serous exudation as the result of pyaemic arthritis, and mares, after parturition, now and again suffer from a similar affection, parturient synovitis.

Inflammation of the lining membrane of the heart (endocarditis) is sometimes associated with an abiding inflammation of the joints.

All these possible causes require to be borne in mind when considering the origin of joint disease. In numerous instances swelling of a joint, resulting from "pyaemic arthritis" or "joint ill" in foals, has at first been referred to a kick from the dam or other form of external violence, and the error has only been recognized when other joints becoming affected rendered the view no longer tenable. This leads us to point out that where disease of the same nature attacks one joint after another in the same animal, some constitutional cause should be suspected, whereas disease confined to one joint only will usually be found to result from accident.