oris and tensor fascia lata.

While the leg is supporting weight the stifle joint is fixed in position
mainly by the quadriceps femoris group of muscles which are attached to
the patella. Tendinous fibres intersect this muscular mass and relieve
muscular strain during weight bearing. Because of the manner in which
the patella functionates with the trochlea of the femur, comparatively
little energy is required to prevent further flexion of the stifle
joint. The patella, according to Strangeways, may be considered a
sesamoid bone.

[Illustration: Fig. 45--Left stifle joint; front view. The capsules are
removed. 1. Middle patellar ligament. 2. Stump of fascia lata. 3. Stump
of common tendon of extensor longus and peroneus tertius. (From Sisson's
"Anatomy of Domestic Animals.")]

The quadriceps group of muscles is assisted by the anterior digital
extensor (extensor pedis) peroneus tertius and tibialis anticus (flexor
metatarsi) muscles. The latter pair (flexor metatarsi, muscular and
tendinous portions, because of their attachment to the external condyle
of the femur and to the metatarsal bone) are enabled to automatically
flex the tarsal joint when the stifle is flexed.

The hock is kept fixed in position by the gastrocnemius and the
superficial digital flexor (perforatus). The latter structure, which is
chiefly tendinous, originates in the supracondyloid fossa of the femur
and has an insertion to the summit of the fibular tarsal (calcis) bone.
It relieves the gastrocnemius of muscular strain during weight bearing.

Smith[35] styles the function of the stifle and hock joints a
reciprocating action, and we quote from this authority the following:

From what has been said, it is evident that flexion and extension
of stifle and hock are identical in their action. When the stifle
is extended, the hock is automatically extended, nor can it under
any circumstances flex without the previous flexion of the stifle.
There is no parallel to this in the body. The two joints, though
far apart, act as one, and they are locked by the drawing up of the
patella, and in no other way. The so-called dislocation of the
stifle in the horse is a misnomer. That the patella is capable of
being dislocated is beyond doubt, but the ordinary condition
described under that term, when the stifle and hock are rigid while
the foot is turned back with its wall on the ground, is nothing
more