The HIP JOINT, like the shoulder, is a ball and socket, but does not
allow such free movement; this is due to the fact that the socket or
acetabulum is deeper than the glenoid cavity and that the capsule is not
so lax. At the same time the loss of mobility is made up for by
increased strength. The capsule has three thickened bands, of which the
most important is the _ilio-femoral_ or _Y-shaped ligament of Bigelow_.
The stalk of the Y is attached to the anterior inferior spine of the
ilium, while the two limbs are fastened to the upper and lower parts of
the spiral line of the femur. The ligament is so strong that it hardly
ever ruptures in a dislocation of the hip. As a plumb-line, dropped from
the centre of gravity of the body, passes behind the centre of the hip
joint, this ligament, lying as it does in front of the joint, takes the
strain in Man's erect position. The other two thickened parts of the
capsule are known as _pubo-femoral_ and _ischio-femoral_, from their
attachments. Inside the capsule, and deepening the margin of the
acetabulum, is a fibrous rim known as the _cotyloid ligament_, which
grips the spherical head of the femur and is continued across the
cotyloid notch as the _transverse ligament_. The floor of the acetabulum
has a horseshoe-shaped surface of articular cartilage, concave downward,
and, occupying the "frog" of the horse's hoof, is a mass of fat called
the _Haversian pad_. Attached to the inner margin of the horseshoe, and
to the transverse ligament where that is deficient, is a reflexion of
synovial membrane which forms a covering for the pad and is continued as
a tube to the depression on the head of the femur called the _fossa
capitis_. This reflexion carries blood-vessels and nerves to the femur,
and also contains fibrous tissue from outside the joint. It is known as
the _ligamentum teres_.
_Comparative Anatomy._--Bland Sutton regards the _ilio-femoral
ligament_ as an altered muscle, the scansorius, though against this is
the fact that, in those cases in which a scansorius is present in Man,
the ligament is as strong as usual, and indeed, if it were not there
in these cases, the erect position would be difficult to maintain. He
also looks upon the _ligamentum teres_ as the divorced tendon of the
pectineus muscle. The subject requires much more investigation, but
there is every reason to believe that it is a tendon which has sunk
into the joint, though whether that of the pectineus is doubtful,
since the intra-capsular tendon comes from the ischium in Reptiles. In
many Mammals, and among them the Orang, there is no ligamentum teres.
In others, such as the Armadillo, the structure has not sunk right
into the joint, but is connected with the pubo-femoral part of the
capsule.
The KNEE JOINT is a hinge formed by the condyles and trochlea of the
femur, the patella, and the head of the tibia. The capsule is formed in
front by the ligamentum patellae, and on each side special bands form
the lateral ligaments. On the outer side there are two of these: the
anterior or _long external lateral ligament_ is a round cord running
from the external condyle to the head of the fibula, while the posterior
is slighter and passes from the same place to the styloid process of the
fibula. The _internal lateral ligament_ is a flat band which runs from
the inner condyle of the femur to the internal surface of the tibia some
two inches below the level of the knee joint. The posterior part of the
capsule is strengthened by an oblique bundle of fibres running upward
and outward from the semimembranosus tendon, and called the _posterior
ligament of Winslow_.
The intra-articular structures are numerous and interesting. Passing
from the head of the tibia, in front and behind the spine, are the
_anterior_ and _posterior crucial ligaments_; the former is attached to
the outer side of the intercondylar notch above, and the latter to the
inner side. These two ligaments cross like an X. The _semilunar
fibro-cartilages_--external and internal--are partial menisci, each of
which has an anterior and a posterior cornu by which they are attached
to the head of the tibia in front and behind the spine. They are also
attached round the margin of the tibial head by a _coronary ligament_,
but the external one is more movable than the internal, and this perhaps
accounts for its coronary ligament being less often ruptured and the
cartilage displaced than the inner one is. In addition to these the
external cartilage has a fibrous band, called the _ligament of
Wrisberg_, which runs up to the femur just behind the posterior crucial
ligament. The external cartilage is broader, and forms more of a circle
than the internal. The synovial cavity of the knee runs up, deep to the
extensor muscles of the thigh, for about two inches above the top of the
patella, forming the _bursa suprapatellaris_. At the lower part of the
patella it covers a pad of fat, which lies between the ligamentum
patellae and the front of the head of the tibia, and is carried up as a
narrow tube to the lower margin of the trochlear surface of the femur.
This prolongation is known as the _ligamentum mucosum_, and from the
sides of its base spring two lateral folds called the _ligamenta
alaria_. The tendon of the popliteus muscle is an intra-capsular
structure, and is therefore covered with a synovial sheath. There are a
large number of bursae near the knee joint, one of which, common to the
inner head of the gastrocnemius and the semimembranosus, often
communicates with the joint. The hinge movement of the knee is
accompanied by a small amount of external rotation at the end of
extension, and a compensatory internal rotation during flexion. This
slight twist is enough to tighten up almost all the ligaments so that
they may take a share in resisting over-extension, because, in the erect
position, a vertical line from the centre of gravity of the body passes
in front of the knee.
_Comparative Anatomy._--In some Mammals, e.g. Bradypus and
Ornithorhynchus, the knee is divided into three parts, two
condylo-tibial and one trochleo-patellar, by synovial folds which in
Man are represented by the ligamentum mucosum. In a typical Mammal the
external _semilunar cartilage_ is attached by its posterior horn to
the internal condyle of the femur only, and this explains the
_ligament of Wrisberg_ already mentioned. In the Monkeys and
anthropoid Apes this cartilage is circular. The _semilunar cartilages_
first appear in the Amphibia, and, according to B. Sutton, are derived
from muscles which are drawn into the joint. When only one kind of
movement (hinge) is allowed, as in the fruit bat, the cartilages are
not found. In most Mammals the superior tibio-fibular joint
communicates with the knee.
The _tibio-fibular articulations_ resemble the radio-ulnar in position
but are much less movable. The superior in Man is usually cut off i
from the knee and is a gliding joint; the middle is the interosseous
membrane, while the lower has been already used as an example of a
syndesmosis or fibrous half joint.
The ANKLE JOINT is a hinge, the astragalus being received into a lateral
arch formed by the lower ends of the tibia and fibula. Backward
dislocation is prevented by the articular surface of the astragalus
being broader in front than behind. The anterior and posterior parts of
the capsule are feeble, but the lateral ligaments are very strong, the
external consisting of three separate fasciculi which bind the fibula to
the astragalus and calcaneum. To avoid confusion it is best to speak of
the movements of the ankle as dorsal and plantar flexion.
[Illustration: (From D. Hepburn, _Cunningham's Text-book of Anatomy_.)
FIG. 7.--Dissection of the Knee-joint from the front: Patella thrown
down.]
The _tarsal joints_ resemble the carpal in being gliding articulations.
There are two between the astragalus and calcaneum, and at these
inversion and eversion of the foot largely occur. The inner arch of the
foot is maintained by a very important ligament called the
_calcaneo-navicular_ or _spring ligament_; it connects the sustentaculum
tali of the calcaneum with the navicular, and upon it the head of the
astragalus rests. When it becomes stretched, flat-foot results. The
tarsal bones are connected by dorsal, plantar and interosseous
ligaments. The _long_ and _short calcaneocuboid_ are plantar ligaments
of special importance, and maintain the outer arch of the foot.
The _tarso-metatarsal_, _metatarso-phalangeal_ and _interphalangeal
joints_ closely resemble those of the hand, except that the
tarso-metatarsal joint of the great toe is not saddle-shaped.
_Comparative Anatomy._--The anterior fasciculus of the external
lateral ligament of the ankle is only found in Man, and is probably an
adaptation to the erect position. In animals with a long foot, such as
the Ungulates and the Kangaroo, the lateral ligaments of the ankle are
in the form of an X, to give greater protection against lateral
movement. In certain marsupials a fibro-cartilage is developed between
the external malleolus and the astragalus, and its origin from the
deeper fibres of the external lateral ligament of the ankle can be
traced. These animals have a rotatory movement of the fibula on its
long axis, in addition to the hinge movement of the ankle.
For further details of joints see R. Fick, _Handbuch der Gelenke_
(Jena, 1904); H. Morris, _Anatomy of the Joints_ (London, 1879);
Quain's, Gray's and Cunningham's _Text-books of Anatomy_; J. Bland
Sutton, _Ligaments, their Nature and Morphology_ (London, 1902); F. G.
Parsons, "Hunterian Lectures on the Joints of Mammals," _Journ. Anat.
& Phys._, xxxiv. 41 and 301. (F. G. P.)