The thigh-bone exposes the largest extent of surface in the knee-joint ; that of the tibia is the next in size. Its superior extremity is ex panded into the same kind of cancellated struc ture as the os femoris possesses at its lower part ; and the width from side to side equals that of the condyles, which rest upon its upper surface. That surface is nearly horizontal, in the erect position of the body ; it is irregularly oval, the long axis passing from side to side, and is marked in the centre by a rough promi nence or spine, in front of which is a depression, and at the back part a notch. On each side of these inequalities there is a smooth articular surface; the inner one the larger, especially from before backwards, and slightly concave ; while the outer one is flat round the margin and raised at the inner side by the base of the spine. Viewing the bone in its anterior aspect we observe that below the articular surface it slopes downwards and forwards to the tubercle which stands out at the upper part of the shin or crest of the tibia; this tubercle gives insertion at its lower part to the strong ligament of the patella, a bursa being interposed between that ligament and its upper smooth portion. Nu merous foramina are to be observed round the head of the bone for the purposes of its nutri tion. Below the tubercle a section of the tibia shows it much reduced in size and somewhat triangular in shape ; the outer side forming, in conjunction with the fibula, a large fossa for the tibialis anticus and other muscles ; and the inner, facing also anteriorly, being a portion of that surface of bone which is covered only by skin and periosteum; except at its upper part, where three flat tendons pass upon it to be inserted by the side of the tubercle in the follow ing order; that of the semitendinosus lowest, the gracilis next above, and the sartorius the highest up and most anteriorly. The posterior surface of the tibia at the supposed place of section has advanced considerably forwards, so as to leave a hollow for the popliteus muscle which lies obliquely on this part of the bone. A few lines below the great articulating surface on the head of the tibia there are two things to be noticed on its posterior aspect; at the outer side a small articular surface for the head of the fibula, and at the inner side a shallow pit where the tendon of the semimembranosus is inserted.
The patella is a flat disk of bone placed in front of the knee-joint ; it equals in width the trochlea of the os femoris to which it is applied, the posterior surface being for that purpose covered with cartilage and divided into two slight cavities by a prominent vertical line ; the articular surface is oval from side to side and does not reach to the lower edge of the bone. Anteriorly, the patella is convex, and its hori zontal slightly exceeds its vertical measure ment, particularly in the female ; into its upper edge are inserted the united tendons of the rectus, cruralis, and vasti muscles ; into its lower edge the strong ligamentum patellae which joins it to the tubercle of the tibia ; it is covered only by skin, fascia, and some ten dinous fibres, to which latter may be attributed the appearance of vertical striae observable on the bone.
(b.) Cartilages, ligaments, 4.c.--The whole of the bony surfaces which come into contact with each other or with the interarticular carti lages during the movements of the knee-joint are covered with " cartilages of incrustation " (see ARTICULATION); and the extent of these on the condyles and trochlea of the os femoris, on the head of the tibia and the posterior sur face of the patella, is well marked even in the dry bones by their smooth and compact ap pearance and the total absence of foramina on the parts so covered. Besides these pure car tilages there are two fibro-cartilages of a semi lunar form lying upon the head of the tibia, which serve to deepen the articulating surfaces for the reception of the condyles. These semi-.
lnnar cartilages, (cartilagines falcate, s. lu nate) as they are named, are thickest at their convex edges which are attached rather loosely to the circumference of the head of the tibia ; the concave edges are thin and sharp, and lie unattached between the condyles and the tibia. The two semilunar cartilages differ slightly from each other in the two following points ; the inner one is falciform, decreasing in breadth from behind forwards ; the greatest width being at the inner and back part, five-eighths of an inch, whilst in front it is hardly more than one quarter of an inch ; the anterior and posterior cornua are separated to the distance of an inch, whilst those of the outer semilunar cartilage approach to within three-eighths of an inch of each other; and, besides that the ring is thus more nearly completed, the breadth of the outer one is more uniform, being about three-eighths of an inch throughout the greater part. The thickness of either of them barely exceeds one sixth of an inch, at the outer margin or thickest part. They are both composed of concentric fibres, the extremities of which are fixed to the central parts of the head of the tibia, before and behind the crucial ligaments, with whose fibres they intermingle; the anterior extremities are usually joined together by a transverse liga ment, but this is sometimes wanting.
The ligamentum Meth; of vast importance in the actions of the knee-joint, is yet the most distant from its articular surfaces ; it extends, broad and flat, from the lower somewhat point ed portion of the patella to the inferior part of the tubercle of the tibia, being in the adult about two inches in length. It forms a strong inelastic but inflexible bond of union of the patella with the tibia, and may with propriety be looked upon as a continuation of the ex tensor tendons which are inserted into the upper and lateral margins of the former bone; some fibres indeed pass over its anterior sur face, but it is only through this bone and its ligament that the extensor muscles can act upon the leg. The patella is thus seen to be placed in a situation analogous to that of the sesamoid bones, in the tendons which play over bony surfaces, in the hands and feet. The ligament of the patella is covered anteriorly by dense integument, and the fascia of the leg : posteriorly a cushion of fat is interposed be tween it and the joint at the upper part, while below it is separated from the bone by a bursa, whose situation was pointed out in the descrip tion of the tibia. (Seefig. 111, b, vol. i. p. 252.) More closely applied to the joint are the lateral ligaments, the posterior and the crucial li6a ments;andportic s of the synovialcapsule which are described by some anatomists as afar and mucous ligaments. The lateral ligaments have a vertical direction at each side of the knee, and are placed nearer to the posterior than the ante rior boundary of the joint ; the upper attach ment is in fact to the tuberosity at the centre of the smaller curve which the articular sur faces of the condyles form at their back part. The internal lateral ligament descends from the tuberosity of the internal condyle of the os femoris to beneath the head of the tibia ; it is nearly three inches in length, of a flattened form, narrow at its commencement, but en larging considerably opposite the joint, to the synovial membrane of which as well as to the internal semilunar cartilage it adheres; infe riorly it again contracts in width. Its upper attachment is covered by the fascia lata; below, it is inserted into the shaft of the tibia just beneath the head of the bone, and anterior to its inner angle ; and the tendons of the sarto rius, gracilis, and semitendinosus cross over it.