Behind the upper half of the ligamentum patellæ is the synovial membrane of the knee-joint; behind the lower half is a synovial bursa and a pad of fat. It is well to remember this in cases of injury to the ligamentum patellæ.
109. Patellar bursa.—The patellar or housemaids’ bursa is situated not only over the patella, but over the upper part of the ligament. This is plain enough when the bursa becomes enlarged. There is another subcutaneous bursa over the insertion of the ligament into the tubercle of the tibia. This is quite independent of the deep bursa between the tendon and the bone.
110. Synovial membrane of knee.—The synovial membrane of the knee, when the joint is extended, rises like a cul-de-sac above the upper border of the patella about two inches. It ascends, too, a little higher under the vastus internus than the vastus externus—a fact very manifest when the joint is distended. When the knee is bent this cul-de-sac is drawn down—hence the rule of bending the knee in operations near the lower end of the femur.
The lower level of the synovial membrane of the knee is just above the level of the upper part of the head of the fibula. The tibio-fibular synovial membrane is, with rare exceptions, independent of that of the knee.
111. Popliteal tendons.—The tendons forming the boundaries of the popliteal space can be distinctly felt when the muscles which bend the knee are acting. On the outer side, we have the biceps running down to the head of the fibula. On the inner side we feel three tendons, disposed as follows:—nearest to the middle of the popliteal space is the semitendinosus, very salient and traceable high up the thigh; next comes the thick round tendon of the semimembranosus; still more internally is the gracilis. The sartorius, which forms a graceful muscular prominence on the inner side of the knee, does not become tendinous until it gets below it.
112. Popliteal bursa.—The precise position of the bursa in the popliteal space, which sometimes enlarges to the size of a hen’s egg, is between the tendon of the inner head of the gastrocnemius and the tendon of the semimembranosus, just where they rub one against the other. The bursa is from one and a half to two inches long. When enlarged, it makes a swelling on the inner side of the popliteal space, which bulges and becomes tense when the knee is extended, and vice versâ. I examined 150 bodies with a view to ascertain how often this bursa communicates with the synovial membrane of the knee. There was a communication about once in five instances. This should make us cautious in interfering too roughly with the bursa when enlarged.
113. Popliteal artery.—The popliteal artery can be felt beating and can be compressed against the back of the femur, close to which it lies. But pressure, sufficient to stop the blood, should be firm, and should be made against the bone nearer to the inner than the outer hamstrings. The line of the artery corresponds with the middle of the ham. It lies under cover of the fleshy belly of the semimembranosus, and the outer border of this muscle is the guide to it. An incision down the middle of the ham would fall in with the vessel just above the condyles.
114. Peroneal nerve.—The peroneal nerve runs parallel with and close to the inner border of the tendon of the biceps. It can be felt in thin persons. There is a risk of dividing it in tenotomy of the biceps, unless the knife be carefully introduced from within outwards. Below the knee the nerve can be felt close to the fibula just below the head, and when pressed upon in this situation causes a sensation to run down its branches to the foot.
THE LEG AND ANKLE.
115. Bony points.—The tubercle of the tibia (for the attachment of the ligamentum patellæ), the sharp front edge called the shin, and the broad flat subcutaneous surface of the bone can be felt all the way down. The inner edge can be felt too, but not so plainly. The lower third is the narrowest part of the bone and the most frequent seat of fracture.