Bauhaus-Universität Weimar

The Cyclopaedia of Anatomy and Physiology, vol. 3: Ins-Pla
Todd, Robert Bentley
4 7 
joint, and there are good reasons for affirming 
that it is continued in a highly attenuated state 
over all the interarticular and incrusting 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 trochléa 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, 
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 patellæ; 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


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