(c.) The mechanical functions of this joint, or the movements of which it is capable within certain limits, and the resistance which it op poses to motion beyond those limits, are plainly deducible from a knowledge of the parts of which it. is composed. To say that the knee is . a hinge-joint with a slight arthrodial or sliding motion, gives a very faint idea of the complex problem which has been solved in its construc tion : to procure firmness without the aid of bony processes interlocking with one another (as in the ankle and elbow); and yet to com bine free power of flexion with impossibility of over-extension ; to oppose large surfaces of bone to one another, so as to ensure stability in the erect posture, without making the joint unsightly by its size, are some of the indica tions most admirably fulfilled.
In the straight or extended position of the leg, the joint is firmly locked so as to admit of no lateral or rotatory motion ; the pointing of the toes in and out in this position is effected by moving the hip-joint. The portions of the condyles forming the segments of large circles are, during complete extension, applied to the tibia and form a broad surface of support; the patella is drawn to the upper or deepest part of the trochlea; and the lateral and crucial ligaments, being attached nearer to the poste rior than to the anterior surface of the thigh bone, are together with the posterior ligament put upon the stretch. If the curve of the arti cular surfaces of the condyles had been uni form, with the lateral and crucial ligaments fixed to the centre of that curve, the posterior ligament only could have acted to restrain the leg from being flexed forwards upon the thigh, and it would be quite insufficient for that pur pose: whereas, by the present arrangement, the centre of motion being placed nearer to the posterior surface of the condyles, the lateral and crucial ligaments cooperate with the pos terior in opposing a strong check to over extension. In flexion the joint admits of mo tion to the extent of about 140 degrees, when it is arrested by the crucial ligaments. During this movement the condyles offer a diminishing surface to the head of the tibia, and the semi lunar cartilages have their ends brought closer together, so as to deepen the cavities for their reception: in extension, the reverse takes place, the semilunar cartilages are pressed out from betwixt the bones to their greatest extent. The adjustment of these fibro-cartilages during flexion is effected partly by their elastic power of resuming their shape when pressure is re moved, and in some degree by the atmospheric pressure urging these moveable parts between the ends of the bones, to prevent the formation of a vacuity in the joint. During the motions of the knee, the patella undergoes important changes of relative position both with regard to the os femoris and the tibia; it plays over the whole extent of the trochlea, being drawn in extreme extension half its diameter above that pulley, whilst in extreme flexion it has moved through a quarter of a circle and is found at right angles with the os femoris, forming in that situation the surface which comes to the ground in kneeling, and so de fends the joint from injury. In relation to the tibia, the patella always keeps the same dis tance from the tubercle, being joined thereto by the ligamentum patellm; but as the con dyles recede during flexion, the patella follows them ; so that a line passing over its anterior surface and that of the tubercle will, if pro longed, reach the point of the great toe, though a similar line in the extended position will fall through the ankle-joint. The necessity for this
advancing and receding movement of the pa tella explains why it is a separate hone instead of forming a process of the tibia, as in the elbow joint the olecranon forms a part of the ulna ; and may also suggest the use of the bursa behind its ligament. It has been said above that besides its ginglymoid motion, the knee-joint has a slight arthrodial motion in the bent position : this is strictly a rotation of the tibia on its axis, and the eflect is to point the toes more or less to the outer side, rotation in wardly to any great extent being prevented by the crucial ligaments. During this movement of rotation, the inner articulating surface on the head of the tibia advances forwards, while at the same time the outer one recedes; the lateral ligaments allow of this motion, in con sequence of their inferior attachments being a good deal below the margin of the joint ; and the crucial ligaments permit the successive, though not the simultaneous, advance or re cession of the cavities on the head of the tibia. In all positions of the joint the arrangement is such that the attempt to thrust forwards the whole head of the tibia is resisted by the ante rior crucial ligament, whilst the posterior pre vents it from being driven backward.* The mucous ligament and fatty body of the joint change their situation in some degree during its motions and may serve to fill spaces which which would otherwise be left vacant: the idea that they are peculiarly concerned in secreting the synovia has been satisfactorily refuted un der " ARTICULATION." The bursa: in the neighbourhood of the knee joint are numerous and not unimportant, from the circumstance that some of them often open into the joint itself. In the layers of fascia anterior to the patella, one or more exist, im perfectly formed and very liable to inflame and suppurate. The situations in which the more perfect specimens are found are as follow, viz. behind the ligament of the patella; between the cruralis and fore-part of the os femoris; beneath the internal lateral ligament ; at the insertion of the semitendinosus, gracilis, and sartorius ; underneath each head of the gernel lus; and around the tendons of the semi-mem branosus and popliteus respectively; the last named bursa is continued down some distance between the popliteus and the tibia, and it often communicates below with the superior tibio-fibular articulation, as well as with the knee-joint above. This joint is supplied with blood from the popliteal artery by five different branches, viz.two superior articular, which wind round the lower part of the os femoris; one mid dle articular which passes through the posterior ligament to the central parts of the joint ; and two inferior articular, which take their course round the head of the tibia, and anastomose freely with each other, and with the two upper; the returning veins go to the popliteal.
The comparative anatomy of the knee-joint gives for the most part the same essential struc ture as we have described in man, though va riously modified.
In most animals, when in a standing pos ture, the knee maintains habitually a state of flexion, and this arrangement conduces much to fleetness and agility of motion. In the ele phant, however, the bones of the hind leg form an upright pillar of support, and the knee is ex tended as in the human subject. The elephant also resembles man in the circumstance of the knee being brought to the ground in kneeling ; whereas, in most genera, the true knee is placed much nearer to the body of the animal.
(Alfred Higginson.)