1. Situated in the subcutaneous tissue over the lower part of the patella and upper part of the ligamentum patellae is the prepatellar bursa, which is the largest subcutaneous bursa in the body. It is frequenty subdivided by septa, sometimes into superficial and deep compartments. From its position it is exposed to injury and infection, giving rise to acute bursitis, while a chronic bursitis commonly occurs from pressure of kneeling, the condition here being known as housemaid's knee. In the acute form, if untreated, the bursa frequently ruptures, and the septic matter invades the subcutaneous tissues in front of the knee-joint, the subsequent swelling simulating a synovitis of the knee-joint. Both forms are generally painful, particularly the acute, and as the bursa lies in close contact with the patella, the infection has occasionally spread to the bone.

2. There is also a small bursa, situated between the patellar ligament and the head of the tibia. Above the bursa the ligamentum patellae rests on an infrapatellar pad of fat, which separates both it and the bursa from the synovial membrane. Occasionally, however, the bursa communicates with the joint. This bursa is more painful when inflamed, owing to the compression to which it is subjected. The pad of fat projecting from underneath the ligament might occasionally be mistaken for an enlarged bursa.

3. A small bursa occasionally exists in front of the tubercle of the tibia, and is noted especially in those who require to kneel (praetibial bursa).

4. Bursae exist between the internal lateral aspect of the head of the tibia, and (a) the sartorius and (b) the gracilis and semitendinosus, which have one in common, while (c) a bursa separates the tendon of the gracilis from the sartorius. These bursae, when enlarged, present an oval fluctuant swelling in the position indicated, (d) The semimembranosus also lies on a bursa close to its insertion ; this, however, is a posterior relation.

5. A bursa exists between the quadriceps tendon and the shaft of the femur, which sometimes communicates with the joint.

The posterior aspect of the knee is occupied by the popliteal space-a lozenge-shaped area-which again may be divided into an upper femoral and a lower tibial triangle. The femoral triangle is bounded by the biceps externally, and the semimembranosus and semitendinosus internally (the former lying under, and on the outer or triangular aspect of the latter). The insertions of these muscles embrace the two heads of the gastrocnemius, which form the boundaries of the tibial triangle. When examining the popliteal space, the knee should be slightly flexed in order to relax the superficial tissues and muscles, and then the finger may detect the trangular area at the back of the femur (which is often affected in acute osteomyelitis), the vessels, nerve, etc. The skin here is less movable than in front, and may become markedly contracted by cicatrices, resulting in flexion of the knee. Contractions forcibly overcome have caused rupture of the skin in this region. The deep popliteal fascia is a continuation of the fascia lata above, and is continuous with the fascia of the leg below. While it does not possess any bony attachment it presents a firm unyielding barrier to tumours, abscesses, etc. which consequently are generally associated with severe pain. Abscesses may attain a large size, containing over a pint of pus, and, unable to escape, generally extend up into the thigh or down into the leg, but may penetrate the popliteal artery or even the joint, while pus may reach the space from the pelvis by following the great sciatic nerve, or from the thigh along the femoral vessels. The popliteal fascia is perforated near the lower part of the popliteal space by the external or short saphenous vein which runs up from the outer side of the foot, and a lymphatic gland generally lies near this point under the deep fascia. A varicose condition of the short saphenous vein has been supposed to be due to narrowness of its opening in the deep fascia. The popliteal space is occupied, particularly in the femoral triangle, by fatty tissue in which the vessels, etc., are embedded.

The sciatic nerve is the most superficial of the important structures. It divides at the upper angle of the space into internal (tibial) and external (peroneal) popliteal nerves. The internal popliteal nerve runs vertically downwards across the space immediately below the deep fascia, and, as the vessels run obliquely across the space, the nerve lies first to their outer and ultimately to their inner side, while at the level of the intercondyloid notch it lies directly superficial to the vessels. The external popliteal nerve runs along the inner border of the biceps tendon to its insertion, under the deep fascia, and then, entering a groove between the soleus and peroneus longus, curves forward between the peroneus and neck of the fibula, 1 inch below its head.

The popliteal artery enters the space through the tendinous arch of the adductor magnus, and first lies on the outer border of the semimembranosus, but then, inclining outwards, reaches the middle line at the level of the intercondyloid notch, which mesial position it maintains. It is separated by a little fatty tissue from the posterior surface of the femur, and lower down is in close relationship to the posterior ligament of the joint and the fascia covering the popliteus muscle. Below the popliteal space the artery divides into anterior and posterior tibial arteries, while in the space it gives off muscular branches to the hamstrings, and two large ones (inferior sural) to the two heads of the gastrocnemius, and five articular branches to the knee-joint.

The two superior articular branches (external and internal) and two inferior articular branches run round to the front of the femur and tibia respectively, where they take part in an anastomosis with the descending branch of the circumflex, the anastomotica magna, and with the recurrent branch of the anterior tibial. Together they form three arches, one at the upper border of the patella, and the other two running transversely below the ligamentum patellae, forming a network surrounding the patella. The fifth, the azygos branch, pierces the posterior ligament, and is distributed to the synovial membrane, crucial ligaments, etc., of the joint. It is accompanied by the geniculate branch of the obturator nerve, and an articular branch of the internal popliteal nerve. The greater part of the popliteal artery is covered by the muscles, about 1 inch in the centre being covered only by superficial tissues. When the limb is extended, the vessel is straight, but it becomes sinuous when the limb is flexed, and its flow of blood is practically stopped on acute flexion. Owing to its deep position the vessel is rarely wounded, but it has been wounded in the lower extremity of the space from the front, the instrument passing through the interosseous space, and it has been ruptured by external violence. Save for the thoracic aorta, this vessel is the one most frequently affected by aneurism, due probably to movement (which, when excessive, may damage the inner and middle coats), to the laxity and small amount of support given by the surrounding tissues, and to the vessel breaking up into large branches just beyond this point. Such aneurisms may attack the bone, and give rise to joint symptoms ; may press on the nerves ; may impede the venous return from interference with the vein, or may point posteriorly. The vessel is best ligatured at the upper and inner portion of the space through an incision some 4 inches long, made parallel to and just behind the tendon of the adductor magnus, the limb being flexed, abducted, and rotated outwards. The sartorius is drawn backwards, the adductor magnus forwards, the semimembranosus backwards, and then the vein is carefully detached from the artery, and the ligature passed. The collateral circulation is abundant through the descending branch of the external circumflex with perforating of deep femoral and anastomotica magna of superficial femoral, and these again with the articular branches of the popliteal and tibial recurrents.

Emboli are particularly apt to lodge at the bifurcation of the popliteal, gangrene of the leg frequently following from blockage of the three main vessels. The vein possesses an unusually thick wall, and is intimately associated with the artery, lying first to its outer side, then directly behind, and, finally, to the inner side. The vein generally escapes injury from external violence, although more superficial.

The lymphatic glands of the space, some five in number, lie embedded in fatty tissue in close proximity to the vessels, with the exception of one already mentioned, which lies close to the fascia lata and the short saphenous vein. They receive the lymphatic channels from the sole of the foot and back of the leg-those which accompany the short saphenous vein and from the anterior tibial gland. The efferent vessels run to the deep femoral glands within the saphenous opening. Popliteal abscess is often due to suppuration of these glands. It also is frequently caused by osteomyelitis of the lower end of the femur, the pus coming to the surface in the region of the triangular area on the posterior surface of the shaft.

Flexion of the knee-joint from disease of the joint may in part be due to contraction of the hamstring muscles from irritation, the muscles being supplied from the fifth lumbar segment through the great sciatic, which also supplies the joint in part. In some cases, not merely flexion, but subluxation, may be produced from drawing of the tibia backwards. On the other hand, these muscles frequently become permanently contracted in flexion of the knee, and necessitate tenotomy. In subcutaneous tenotomy of the biceps there is risk of wounding the external popliteal nerve, and therefore it is frequently better to do an open operation. Contraction of the muscle, or putting it on the stretch by extending the leg, increases its distance from the nerve, while rendering the tendon more superficial, and then the tenotome is introduced between the nerve and the tendon, and the latter cut by bringing the tenotome toward the skin.

There are numerous burs.il in the popliteal region : (i) The largest is situated between the internal condyle and the inner head of the gastrocnemius and semimembranosus. In adult life it frequently communicates with the joint, and may become markedly enlarged, presenting a firm swelling on extension, which may disappear on flexion with steady pressure. Such a swelling may receive transmitted impulse from the femoral artery, and so resemble aneurism, while it frequently interferes with the movements of the joint, necessitating removal. (2) A small bursa, between the outer head of the gastrocnemius and the condyle. (3) One between the popliteus tendon and the external lateral ligament, not communicating with the joint. (4) One between the popliteus tendon and the outer tuberosity of the femur, which communicates with the joint, and frequently also communicates with the tibio-fibular articulation, which thereby may communicate with the knee-joint. (5) One between the biceps tendon and the external lateral ligament, which, when enlarged, may press on the peroneal nerve, and cause pain. (6) A small one between the semimembranosus tendon and the back of the head of the tibia, already mentioned. It does not communicate directly with the joint, but may do so indirectly through communication with (1).