1896. ] ANATOMY OF PETROGALE XANTHOPUS. 691 
fabella articulates with the head of the fibula. The internal 
lateral ligament extends down the inner side of the head of the 
tibia for about an inch below the level of the joint; its anterior 
fibres are attached highest and its posterior lowest. The two 
crucial ligaments are strong and have the usual attachments. The 
external semilunar cartilage is attached anteriorly to the tibia just 
behind the anterior crucial; posteriorly it turns up to be attached 
to the posterior part of the external surface of the internal condyle 
some way behind the attachment of the posterior crucial ligament. 
Externally the popliteus tendon is connected to the convex margin 
of the cartilage. Besides the above-named posterior attachment 
of the external semilunar cartilage there is a narrow band which 
binds it feebly to the posterior margin of the head of the tibia, 
The synovial membrane of the joint is continued upward under 
the quadriceps tendon for about half an inch above the top of 
the articular cartilage of the trochlea. The ligamentum muco- 
sum is well marked and contains a pad of fat in its lower part ; 
above it is continued up as a narrow tube of synovial membrane 
to the posterior margin of the trochlea. The internal semilunar 
cartilage is attached anteriorly nearly opposite the external, 
posteriorly it is fixed to the tibia in front of the posterior crucial 
ligament. 
Fig. 5. 
Lower end of femur of Petrogale, with ligaments attached. 
(Same lettering as fig. 4.) 
The T%bio-fibular Articulations.—The lower half of the fibula has 
a concave surface where it is in contact with the tibia and is bound 
to that bone by fairly strong interosseous ligaments. The upper 
half of the fibula allows the head to glide backward and forward on 
the external tuberosity of the tibia for about 3 inch. During 
internal rotation of the leg on the thigh the head of the fibula is 
pressed back by the external condyle of the femur and forms a 
spring-like buffer to check that movement. The superior tibio- 
