NORMAL ANATOMY OF THE KNEE-JOINT. 



47 



joint, and there are good reasons for affirming 

 that it is continued in a highly attenuated state 

 over all the interarticular and incrustmg carti- 

 lages, giving them their smooth and secreting 

 surfaces. (See ARTICULATION.) The remain- 

 der of its extent may be traced in the following 

 manner: from the upper edge of the patella 

 it ascends behind the common extensor tendon, 

 and is loosely reflected upon the thigh-bone 

 two or three inches above the trochlea in the 

 extended position of the limb ; from each side 

 of the patella it passes backwards in a broad 

 sheet, whose lower margin is attached to the 

 edge of the semilunar cartilage, and thence goes 

 to the tibia, while above it is loosely reflected 

 on to the condyles of the os femoris, at the 

 distance of nearly an inch from their cartila- 

 ginous surfaces ; from the back part of the con- 

 dyles these two lateral portions pass into the 

 fossa and join to cover the anterior surface of 

 the crucial ligaments. From the lower edge of 

 the patella the synovial membrane descends to 

 cover the fatty body which is placed in that 

 part of the joint, and it accompanies a small 

 prolongation from that body which frequently 

 passes across the joint to the lowest portion of 

 the trochlea of the os femoris, forming what 

 has been named the mucous ligament; this 

 structure however is not always present. 

 There is some discrepancy in the descrip- 

 tions of different anatomists as to the alar 

 ligaments, which are described as folds at the 

 sides of the patella, and it seems altogether 

 unnecessary to distinguish these lateral portions 

 by name from the other parts of the synovial 

 capsule. They are simply folds of the syno- 

 vial membrane projecting into the articular 

 cavity, and obviously destined to increase the 

 extent of synovial surface for a greater amount 

 of secretion. This membrane has a dense cel- 

 lular tissue on its outer surface, by which it is 

 connected firmly to the posterior surface of the 

 extensor tendons and fascia lata. It possesses 

 some degree of elasticity, but its chief power 

 of accommodation to the motions of the joint 

 is derived from its lax connection with sur- 

 rounding parts. 



(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 



synovial membrane of the knee-joint, to distend it 

 by injecting some coagulating fluid, as size, through 

 a hole bored through the centre of the patella. 

 Mechanik der menschlichen Gehwerkzeuge, p. 195. 

 ED.] 



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 nbro-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 patella?; 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 bone 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 



