Hygromas result from chronic inflammation of serous bursæ, naturally existing, or of serous bursæ which form at prominent points where the skin is exposed to repeated friction, blows, shocks, or over-extension. They usually develop slowly, without producing marked pain or alarming symptoms, and therefore the practitioner is seldom consulted until the swelling has attained a considerable size. The hygroma is usually characterised by its non-painful character and by regular fluctuation throughout. The walls of the serous bursæ are merely thickened, so that palpation is easy. Should the hygroma become infected and inflamed, it assumes the same characters as an abscess: it becomes highly sensitive, is surrounded by œdematous infiltration, shows more marked fluctuation at some specialised point, and eventually breaks, discharging pus. Long-standing hygromas often have thickened fibro-cartilaginous and extremely hard walls, which render examination more difficult. [Illustration: $1] Where the hygroma is much exposed to friction the skin covering it undergoes complete transformation, the layers of epidermis becoming converted into a substance resembling horn. The entire substance of the wall of the hygroma then undergoes change, and is often infiltrated with lime salts or encrusted with plates of bone of varying thickness. HYGROMA OF THE KNEE. This condition is very common in bovines, a fact explained by the manner in which these animals rise. Whilst the hind limbs are being lifted, the entire burden of the body weight is transmitted to the knees and the tissues covering them; so that, if the ground is rough, the skin may be sufficiently displaced to produce laceration of the subcutaneous connective tissue, serous effusion in the layers of connective tissue, and the immediate production of an hygroma beneath the skin and in front of the synovial sheaths of the extensor tendons. Hygroma is principally caused by falls on the knees, roughness of the stable floor, prolonged decubitus during the course of a serious disease, or after an attack of foot-and-mouth disease. Hygromas may be no larger than a turkey’s egg or a man’s clenched fist, but sometimes assume the dimensions of a child’s head. Calcification and ossification of the walls and cornification of the skin are commonest in old hygromas of the knee. The sensibility and uniform fluctuation make mistakes in diagnosis difficult. The condition can only be confused with distension of the synovial sheath of the extensor metacarpi magnus; but this (synovial) swelling extends in the same direction as the tendon, _i.e._, vertically, attains the lower third of the radius, and is broadest above. Hygromas must also be distinguished from tumours. Moussu only mentions a single case of this kind, the tumour being very slightly bosselated and, naturally, revealing no fluctuation.