The periosteum is a tough fibrous coat, which lines the diaphyses of long bones, which acts as a limiting membrane to the bone cells, or osteoblasts, keeping them in bounds, and which supplies the bone with blood, particularly in youth and adolescence, in addition to that supplied by the nutrient artery. Between the periosteum and the shaft is a thin layer of loose connective tissue in which numerous osteoblasts generally lie. It should be noted that the periosteum has no power of forming bone, that function belonging to the osteoblasts. In infancy the periosteum is practically continuous with the synovial membrane of the joint, while in adolescence the periosteum dips in, and becomes intimately connected with the edges of the epiphyseal plates.

To revert, then, to the progress of the pus in osteomyelitis. It spreads in the loose areolar tissue between periosteum and shaft, stripping the periosteum, cutting off the periosteal blood-supply, and frequently killing the bone in mass- necrosis. It also tends to invade the joint from this position, and is generally successful in infancy, owing to the connection between periosteum and synovial membrane ; whereas in adolescence the periosteum, dipping in to fuse with the epiphyseal plate, protects the joint from invasion. Sooner or later the periosteum becomes involved in the pyogenic process-periostitis-is softened and perforated, and the pus forms a superficial abscess, which bursts. It will therefore be seen that if osteomyelitis is to be treated successfully it must be done at once, an exit being given to the pus while it is yet in the medulla of the bone, and before it has done much damage, and come to the surface.

The knee-joint is supplied with 0l00d by the five articular branches of the popliteal already described, and derives its nerve-sfípplv from branches of the nerves to the vasti (anterior crural) in front ; from the internal and external popliteal (great sciatic) laterally and behind, and from the geniculate branch of the obturator posteriorly.

Fluid distension of the knee-joint frequently occurs as a result of injury, the distension in tubercular disease being more commonly due to masses of granulation tissue within the joint. The suprapatellar pouch becomes markedly distended, while the patella is raised from the trochlear surface on a waterbed (floating patella). As in the case of the hip, a position of flexion is that in which the joint accommodates most fluid, and hence a flexion of some 25 degrees is generally met with in cases of distension of the joint from acute or chronic affections, that being the position of maximum capacity. The muscles, supplied by the same nerves as the joint, aid in actively keeping the joint in this position, and night startings-a condition characteristic of joint affections where ulceration of cartilage has occurred to some extent-are due to the relaxation and possibly jerking of these muscles on sleep supervening, producing a jarring of the bones on one another, the delicate nerve fibrils which exist in such abundance just under the articular cartilage being crushed. As articular cartilage does not contain sensory nerve filaments, the normal joint does not give rise to acute pain on jarring, unless excessive. The flexed position if persisted in may become permanent, due to contraction of the muscles and ligaments, and also to anchylosis of the joint, while in other cases, where the joint, including the crucial and other ligaments, is disorganized by disease, a subluxation is liable to occur, the head of the tibia being drawn backwards by the contracted hamstring muscles, while slight eversión is also frequently observed, due probably to the action of the biceps.

Knock-knee, being an affection of the lower third of the femur, and not of the joint, is treated under that heading.

The knee-joint is frequently excised for tubercular disease, generally through a horseshoe incision from one condyle to the other, above the patella, the skin, fasciae, quadriceps, and synovial membrane being cut through, and the joint opened. The diseased synovial tissue is removed, the lower portion of the condyles of the femur and the head of the tibia sawn off, care being taken with regard to the latter not to remove so thick a slice as to imperil the epiphyseal cartilage, and to break rather than cut the last small portion at the back of the tibia, so as to avoid injury to the vessels, the artery lying just behind the posterior ligament. Care must also be taken that the cuts are so made as to leave the limb perfectly straight when the cut surfaces are accurately in apposition. Unlike other joints, the aim in excision of the knee is to secure broad surfaces on both bones, so as to obtain firm osseous anchylosis. While a movable joint can be obtained by excision of the knee, it has been found to be of no practical value, and generally unfit to support the weight of the body.

Dislocations of the knee-joint are rare, are generally due to great and often direct violence, and, as a rule, the joint is opened, the lateral and crucial ligaments being torn when the dislocation is complete. The tibia may be displaced in any direction, the lateral displacement being perhaps more common and less complete. Of the antero-posterior dislocations, the forward displacement of the tibia is the more common, and in this form the vessels and nerves are said to be more damaged by the femur than by the tibia in the backward form. As a matter of fact, the vessels and nerves are apt to be damaged by the severe injury which causes the dislocation, and as a rule the damage to the joint is such that the tibia may be freely moved in any direction, antero-posteriorly or laterally.

Amputation at the knee-joint may be performed by various methods. In Smith's operation two lateral flaps are made, commencing below the tibial tuberosity, and extending downwards for fully 3 inches, that on the inner side being made the larger to accommodate the internal condyle. The flaps consist of skin and cellular tissue alone, and the condyles of the femur are not cut, a disarticulation being performed. Carden, Gritti, Lister and also use skin-flaps, but they remove the condyles. Carden makes an anterior horseshoe from one condyle to the other, crossing the centre of the ligamentum patellae below, and joins the ends of this incision by a posterior transverse one. Gritti makes the same incision, but extends it to the lower border of the tibial tuberosity, and he retains the patella which he attaches, after cutting off the cartilaginous surface to the cut surface of the femur. Lister makes an anterior transverse cut at the level of the tibial tuberosity, and joins the ends of the anterior by a posterior, which descends at an angíe of 45 degrees to the anterior.