i 
THE KNEE-JOINT. 293 
convex margins of the two semilunes, crossing the front part of the non-articular 
area on the tibial head in its course. 
The synovial membrane of the knee-joint is not only the largest, but the most 
elaborately arranged of its kind in the body. It not only lines the capsule, but it 
forms a more or less extensive covering for the intracapsular ligaments and the 
free surface of the infra-patellar pad of fat. This pad acts as a wedge which fits 
into the interval between the patella, tibia, and femoral condyles, and the synovial 
membrane upon its surface forms a band or fold which extends from below the 
level of the patellar articular surface to the anterior part of the intercondyloid 
notch. This is in no sense a ligament, although it 1s named the ligamentum 
mucosum, or plica synovialis patellaris. At its femoral end it is narrow and attenu- 
ated, but at its patellar end it expands laterally to form wing-like fringes or mem- 
branes—the alar ligaments (plicee alares)—which are often distinguished from each 
other as the inner (plica aliformis medialis) and the outer (plica aliformis later alis). 
These folds are more or less loaded with fat. 
Apart from these special foldings, the synovial membrane lines the deep surface 
of the common extensor tendon, arid extends upwards for a variable distance above 
the patella. This extension of the joint-cavity almost always communicates with 
a large bursa situated still higher on the front of the femur. Tracing the synovial 
membrane downwards, it will be found to cover both surfaces of the semilunar fibro- 
cartilages. The peripheral or convex margins of these cartilages are only covered 
by this membrane where they are unattached to the capsule. A prolongation 
invests the intracapsular portion of the tendon of the pophteus muscle, and 
separates this tendon from the back part of the tibial head, besides intervening 
between the external semilune and the head of the tibia. 
From the back part of the joint-cavity the synovial membrane extends forwards, 
and provides a partial covering for the crucial ligaments. 
This somewhat complicated arrangement of the synovial membrane may be 
readily comprehended if it be borne in “inind that it re ally represents the fusion of 
three separate synovial membranes, which in some animals are permanently distinct. 
These are indicated in the two femoro-tibial and the single femoro-patellar parts of 
the articulation. 
The joint-cavity may communicate with burse situated in relation to the inner 
head of the gastrocnemius muscle and the tendon of the semi-membranosus muscle, 
besides the large supra-patellar bursa already described. Lastly, there may he 
intercommunication between this joint-cavity and that of the superior tibio-fibular 
articulation. 
Movements at the Knee-Joint.—In studying the movements which may occur at the 
human knee-joint, it is necessary to bear in mind that the lower hmb of man is primarily required 
for purposes of support and locomotion. The principal requirement of the former function is 
stability accompanied by rigidity, whereas in the latter function the special desideratum is, regu- 
lated and controlled mobility. Thus, in the same joint, two entirely opposite conditions have 
to be provided. The stable conditions of support are chiefly concerned in the maintenance 
of the erect attitude, and the mechanism associated therewith does not call for the exertion of a 
large degree of sustained muscular effort. 
In standing erect the attitude of the limb is that of extension, which mainly concerns the 
femoro-tibial parts of the joint. In this position the force of gravity acts along a vertical line 
which falls in front of the transverse axis of the joint, and therefore any tendency to flexion, @e. 
bending backwards, is mechanically counteracted by the application of a force which tends to 
produce bending forwards (so-called over-extension). This, however, is absolutely prohibited in 
normal states of the joint, by the tension of the posterior and lateral ligaments aided by the 
anterior crucial ligament. The value of this fact may be seen by observing the effect produced by 
giving the jointa ‘sudden push from behind, which causes an immediate rev versal of the positions of 
the transverse and vertical axes, whereby the body weight at once produces flexion of the joint. 
The semilunar cartilages and the infrapatellar pad of fat also assist in maintaining extension, 
by reason of their close adaptation to, and packing round the condyles as these rest upon the tibia. 
The anterior margin of the intercondyloid fossa is also brought into contact with the front of 
the anterior crucial ligament. 
In the position of extension the patella is retained at a high level in relation to the trochlear sur- 
face of the femur, so that the lower articular facets of the patella are in contact with the trochlea. 
During locomotion the movements of the knee-joint are somewhat intricate, for both the 
femoro- tibial and the femoro- patellar sections of the joint are brought into action. The principal 
movement which results is flexion, with which there is associated, both at its beginning and 
