Having studied the upper extremity of the femur with regard to the articulation, proportions, and contour of the region of the hips, we shall continue the study of this bone by examining its shaft and inferior extremity, and this latter part brings us to the articulation of the knee.

The shaft of the femur is not straight, but is distinctly curved, with its convexity forward and outwards. On the living model this convexity may be recognised in the form of the anterior surface of the thigh, which is distinctly convex in front and on the outer side, the muscles which cover the femur in front being disposed in such a manner as to increase this appearance still more, as their fleshy masses are grouped together in the middle line of the anterior region of the thigh. The outward projection is most marked in cases of excessive muscular development, and is consequently more obvious in the male than in the female, and is best shown in the limbs of athletes. Again, the femur, in the model when standing upright, is not directed vertically, but rather obliquely from above downwards and inwards (Fig. 43), so that the superior extremities of the femur are placed at some distance, comparatively speaking, from each other, while the lower extremities come very near each other at the level of the knees. In the female this obliquity is more clearly marked than in the male, for the upper extremities of the two bones are in the former placed more widely apart, as we have already seen when demonstrating the relative diameter of the hips (inter-trochanteric diameter) in the female.

The shaft of the femur presents three surfaces—one anterior, one postero-external, and one postero-internal; and three borders, two lateral and one posterior. The two lateral borders are very rounded, not sharp; the posterior border, on the contrary, is very prominent, and forms a rough line, called the linea aspera (1, Fig. 43), which gives insertion to a number of muscles. This linea aspera divides above into two bifurcations slightly diverging, of which the outer one (2, 2, Fig. 43) proceeds towards the great trochanter (gluteal ridge), and the inner one passes towards the lesser trochanter. Below, the linea aspera bifurcates in the same manner, one of its branches going to the inner, the other to the outer condyle of the femur (4, 4, Fig. 43).

The inferior extremity of the femur is widely expanded, both in the transverse and in the antero-posterior diameter. When we examine the posterior aspect of this extremity (Fig. 43) we see that it is formed by two large prominences directed backwards, which are termed the external and internal condyles. These project laterally, and the inner condyle is much more prominent than the outer. The inferior and posterior surfaces of these condyles are smooth and covered with articular cartilage; between them is a deep hollow behind called the inter-condyloid notch (13, Fig. 43). When we examine the anterior surface of the lower end of the femur we see that the condyles are united, and their smooth and continuous articular surface, covered with cartilage, serves to articulate with the knee-cap. This is the patellar surface. This surface presents a depression in the middle line and two lateral lips, of which the external, continuous with the external condyle, is more prominent and rises higher than the internal, which is continuous with the internal condyle. These details are very important, for, as we shall see, the lips of the patellar surface show prominently beneath the skin when the knee is strongly flexed, and we can notice their differences in prominence and height.

In forming the articulation of the knee, the lower end of the femur is in direct contact with the patella and the upper end of the tibia, and is connected (by ligaments) with the upper end of the fibula. We will now consider the patella and the upper extremities of the two bones of the leg.

The patella, which has been compared to a disc (whence its name), is more nearly triangular in shape, presenting an anterior surface longitudinally striated and slightly convex, and a posterior surface moulded on the patellar surface of the femur, and forming an oval articular surface with a median ridge and two lateral hollows. The borders of the patella are three: two lateral oblique borders for the attachment of muscles and ligaments; and a base directed upwards, into which the tendon of the quadriceps extensor (and particularly the rectus femoris) is inserted. The apex of the bone is directed downwards, and attaches a strong ligament which is inserted into the tubercle of the tibia, and is termed the ligament of the patella. Properly speaking, this ligament is a continuation of the tendon of the rectus femoris muscle. The patella should be considered as a sesamoid bone, an osseous nodule developed in the substance of this tendon.

The leg, like the forearm, is composed of two bones. One, the larger of the two, is placed on the inner side (1, Fig. 44)—the tibia, or shin-bone; the other, much shorter, is situated on the outer side, and a little behind—the fibula (9, Fig. 44). As in the case of the two bones of the forearm, the bones of the leg end at different levels above and below. Above, the tibia rises higher than the fibula, and alone takes a direct part in the articulation of the knee; below, the fibula extends lower than the tibia—so that the outer ankle (external malleolus) descends lower than the inner one. We will now for a moment examine the upper extremities of the two bones.

The upper extremity of the tibia is expanded from side to side, to form the tuberosities (internal and external), surmounted by two articular surfaces, external and internal (2, 3, Fig. 44), for articulation with the corresponding femoral condyles. The non-articular interval between these two surfaces presents in its centre a projection like a bifurcated cone which is known as the spine of the tibia. In front of the spine and behind it are rough triangular surfaces for the attachment of the crucial ligaments.

The circumference of the upper end of the tibia forms a broad rough margin for attachment of the capsule of the knee-joint. In front, it is prolonged downwards into a large triangular surface which ends in a rounded eminence, called the tubercle of the tibia, which gives insertion to the ligament of the patella previously mentioned. On the outer side and back of the external tuberosity is a rounded facet, smooth and covered with cartilage, and directed mainly downwards for articulation with the head of the fibula (10, Fig. 44). Lastly, on the back of the bone is the notch of the tibia, separating the two tuberosities from one another.

The upper end of the fibula forms (10, Fig. 44) an irregular rounded knob. It is situated external to and rather behind the upper end of the tibia, and articulates, as already seen, by its upper surface with the outer tuberosity of the tibia. External to the articular surface is a smooth area for the attachment of the biceps tendon, and the long external lateral ligament of the knee. Behind this area is the styloid process, which gives attachment to the short external lateral ligament of the knee. The head of the fibula can be plainly felt through the skin, below and behind the head of the tibia.

Such are the bony structures which take a direct (femur, tibia, patella) and indirect part (fibula) in the conformation of the knee-joint. In the living model, in the erect posture, the condyles of the femur rest by their inferior surfaces on the upper surface of the tibia, and the apposition of the articular surfaces is rendered more exact by the presence between the articular surfaces of the femur and tibia of two semi-lunar fibro-cartilages, which increase the depth of the tibial surfaces (5, 5, Fig. 46), so that the corresponding condyle of the femur is received into a true articular cavity. At the same time, the patella is closely applied to the confluent anterior surface of the lower end of the femur. When, on the other hand, the model is kneeling, or, more generally, when the leg is flexed (carried backwards), the patella, which is firmly fixed to the tibia by its ligament, glides downwards on the femur, and comes in contact with the lower part of its anterior surface at the same time that the posterior parts of the condyles in turn roll over the upper surfaces of the tibia.

After this rapid sketch of the bones which enter into the formation of the knee-joint, we have now to study in detail its ligamentous apparatus. The ligamentous apparatus of the knee is essentially composed of a fibrous capsule, attached to the borders of the articular surfaces of the femur, patella, and tibia. It is strengthened on all sides by the tendons of muscles attached to these bones in the neighbourhood of the knee. Without entering into unnecessary detail, we must consider the arrangement of this capsule, and its disposition on its anterior, posterior, and lateral aspects, and then consider how it affects the movements of the knee-joint, rendering some movements easy and extensive, while it limits others or makes them almost impossible.

In front (a a, Fig. 45) the capsule is loose and expanded and is mainly formed by the insertion of the quadriceps extensor muscle into the patella, with its continuation to the tubercle of the tibia as the ligament of the patella. This arrangement accounts for the ease and extent of the movement of flexion. In this movement, the tibia being carried backwards, and drawing with it the patella in the relaxed condition of the quadriceps extensor muscle, the capsule of the joint is put on the stretch. It would curtail the movement of flexion if it were short and compact; but the capsule at this point is so expanded and loose that no increase of movement on the part of the leg can cause any tension. Thus flexion of the knee may be prolonged until the calf of the leg comes in contact with the posterior surface of the thigh.

Behind, the capsule of the joint is short and thick, and consists of a strong posterior ligament. When the leg is flexed on the thigh, this ligament is relaxed; but as the leg passes from flexion to extension it becomes tense, and when extension has arrived at that point which brings the leg in direct continuation with the thigh, no further movement is possible in a forward direction, and the limb is therefore fixed in this position.

There are, moreover, other important ligaments which act, and still more forcibly, in the same manner; namely, the internal and external lateral ligaments of the joint, which must now be considered.

The internal lateral ligament is a broad band of fibres, which passes from the inner tuberosity of the femur over the inner tuberosity of the tibia to be attached to the upper part of the shaft of the tibia on the inner side. It helps to form the contour of the inner side of the knee.

The long external lateral ligament is a rounded cord, very strong and quite distinct from the capsule. Its inferior extremity is not inserted into the tibia, but into the head of the fibula between the insertions of the biceps muscle (Figs. 46, 47, a, b). The most remarkable characteristic of these ligaments is that their upper ends, attached to the lateral surfaces of the condyles of the femur, are not inserted at the centre of the curve of the condyles, but at a point situated more posteriorly (a, b, Figs. 46 and 47). During flexion these ligaments are therefore relaxed, but as extension is produced, as the tibia is carried forward on the curved surface of the condyles of the femur, these ligaments gradually become tense, their points of insertion becoming further and further apart, owing to the eccentric insertion of their superior extremities. When the leg reaches that position of extension in which it is in line with the femur, the tension of the lateral ligaments is such as to arrest its movement and completely fix the leg in relation to the thigh. This may be seen by comparing Figs. 46 and 47. We see, then, that owing to the position of the anterior, posterior, and lateral ligaments of the knee-joint, the movement of flexion in this joint is very extensive, while that of extension is limited, as it cannot be prolonged further than the position which brings the leg into direct continuity with the thigh.

These ligaments are also responsible for the amount of lateral movement and rotation possible in the knee-joint. This articulation being formed by two condyles, it is very evident that lateral flexion cannot be produced, for then it would be necessary for one of the condyles to become detached from the corresponding articular surface of the tibia. Lateral movement is thus impossible, but a slight gliding movement under certain conditions takes place between the condyle of the femur and the tibia, a movement produced by a rotatory movement, of which the other condyle forms the centre. These slight movements of rotation, which contribute to the motion by which we direct the point of the foot outwards or inwards, are impossible during complete extension, when the tibia is fixed on the femur by the tension of the lateral ligaments and the posterior part of the capsule, and all the parts have already arrived at their maximum of tension. But when flexion occurs, and especially when the leg is at a right angle to the thigh, as in the seated subject, slight movements of rotation of the leg become possible; they are of small extent, it is true, especially inwards; since it is easier for the knee to take part in the movement which turns the foot outwards, than that which turns it inwards. This difference between rotation inwards and outwards is due to the presence within the joint of two ligaments called the crucial ligaments, of which we will only mention that they arise from the upper end of the tibia, between the two articular surfaces (page 136), and, crossing one another, are attached above to the sides of the inter-condyloid notch of the femur on the corresponding surface of each condyle. This crossing of the two ligaments is increased by the rotation of the tibia inwards, as this movement tends to twist them one on the other, and fix the tibia, so as to prevent rotation of the leg inwards. On the other hand, rotation outwards unwinds these ligaments and renders them more relaxed; so that this movement could be very extensive if the lateral ligaments did not prevent too great displacement between the condyle of the femur and the corresponding surface of the tibia.

There are still in relation to the knee-joint several anatomical particulars which must now be examined in relation to the external form of the living model.

The posterior surface of the knee is covered by numerous muscles and tendons which form the boundaries of the ham or popliteal space; the study of this region will therefore be undertaken along with the description of the muscles of the leg and thigh. On the outer and inner surfaces, and on the front of the knee-joint, however, many details of outward form are caused solely by the osseous and ligamentous parts, to which we must now draw particular attention.

The anterior surface of the knee alters in shape in the two positions of extension or slight flexion, and in forcible flexion. In forcible flexion we notice particularly the form of the articular lower end of the femur (see page 134). In extension we find on the anterior surface of the knee, in succession from above downwards, a flat triangular space above the patella, corresponding to the tendon of the rectus femoris muscle; next, the prominence of the patella, showing clearly beneath the skin its triangular shape, with the base above and apex below; the two superior angles of the patella are frequently very distinct in the form of two small rounded projections. Below, the ligament of the patella forms a longitudinal eminence in the middle line, reaching to the tubercle of the tibia, which is seen (in semi-flexion) as a large projection. But, again, we frequently perceive on each side of the ligament of the patella a slight, soft eminence, easily depressed, which corresponds to parts not yet mentioned. These are the thin lateral portions of the capsule (lateral ligaments of the patella), which extend from the sides of the patella and patellar ligament to the tibial tuberosities. These portions of the capsule (c, Fig. 45) are thin, and cover a large mass of adipose tissue, which is prolonged into the interior of the joint (as the alar ligaments), and which forms packing for the articulation on either side of the ligament of the patella (6, Fig. 45). When the quadriceps extensor muscle draws forcibly on the patella and its ligament, these compress this adipose mass to a much greater degree, so as to form a bulging on each side, and then the bilateral prominence in question is much more clearly marked.

On the external lateral surface of the knee we note the osseous prominences of the tubercle of the tibia (seen here in profile) and its external tuberosity; behind and below, the head of the fibula; and above, the external condyle of the femur. To certain of these prominences are attached the tendons coming from the thigh, marked on the external surface of the knee by three strong vertical bands; viz.:—in front, the tendon of the patella (seen here in profile); behind, the tendon of the biceps femoris, proceeding to be inserted into the head of the fibula; and between them the part of the aponeurosis of the deep fascia of the thigh, which becomes narrow and thickened, and forms a true tendon—the ilio-tibial band—attached to the outer tuberosity of the tibia.

The form of the internal aspect of the knee is simple. Here the internal condyle of the femur and the corresponding tuberosity of the tibia form together a large regular hemispherical surface. The internal lateral ligament forms a broad band (page 139), which sweeps over the inner tuberosity of the tibia, accompanied by the tendons of the sartorius, gracilis, and semi-tendinosus muscles, to become inserted into the upper portion of the shaft of the tibia. The ligament and tendons soften the outlines of the bones, and give a rounded contour to the inner side of the knee.