This is an affection of the tendons, ligaments, and bursa connected with the hock. From a strain, violent exercise, or similar cause, an increased action is set up in these parts which glide upon each other, irritation results and the parts enlarge. Or, an exudation of serum occurs, tinged or not with blood, which may be absorbed afterward or remain for a long time a soft movable tumor; or by degrees an ossific or bony deposit takes place, resulting in a firm, hard, bony tumor. These several stages or degrees of the same common affection have been termed _Bog-Spavin_, _Blood-Spavin_, and _Bone-Spavin_. Windgalls and Thorough Pins are but local variations of the same essential condition—enlargements of the bursa and tendons of the joint. It usually shows itself on the inner and lower side of the hock at the lower portion of the joint. A careful comparison of the two legs with the eye and hand will best disclose the evil. “Sometimes it appears as a soft swelling of the femoral vein along the inner surface of the hock. Or, again, as a hot, painful, lameing swelling, extending from the posterior border of the hock downward, which may afterward become a hard, bony tumor, insensible, causing the animal to limp only when making some exertion. Or, the swelling extends along the inner surface of the hock, oblong, a little broader above than below, bony, sometimes involving the entire joint, and occasioning more or less limping. Or a soft round swelling over the whole internal surface of the hock, at first not impeding the horse’s gait, but afterward becoming indurated and causing a rigidity of the hock and consequent lameness. Or a hard osseous swelling at the upper and inner side of the femur, causing a lameness depending upon the extent to which the ligaments of the joints are involved.” At first the animal seems afraid to use one or the other of the hind legs, and a little lameness is noticed on first starting off. Afterward, when returning to the stable after exertion, the horse stands on the toe of the affected limb, and limps considerably on turning around and first commencing to walk; after moving a little, the lameness disappears, and only returns again after he has been standing some time. This will soon be followed by swelling. CAUSES.—There is a predisposition to this affection in some families of horses, and when this predisposition exists, strains, violent exertion, over-work, a blow or other injury, readily develops the spavin. Horses with high legs, from three to seven years of age, are most liable to it.