_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.