THE SKELETON 83 
In the lower Mammals both tibia and fibula articulate with 
the femur, and contribute to the formation of the knee-joint. 
In Man, under advancing phylogenetic development, the weight 
of the body has come to rest on the tibia alone, and the proximal 
end of the fibula has become disconnected from the femur,} 
and has shortened and shifted downwards along the postero- 
external surface of the tibia. 
The human fibula is now an appendage of the tibia, and the 
fact that its degeneration has not gone farther? is accounted 
for partly by its important connection with the heads of certain 
muscles of the leg (especially the peronei), and by the part which 
it plays in the formation of the ankle (external malleolus). 
The external condyle of the tibia varies very much in different 
races. In the lower races it is much more convex than in the 
higher, and this is probably also the case in the oldest prehistoric 
men. This convexity 1s evidently connected with the frequent 
strong flexure of the knee-joint, such as occurs in squatting. 
On the inner border of the distal extremity of the tibia 
(malleolus internus) there is, in the lower races, a special facet 
which articulates with the neck of the astragalus ; and the presence 
of this may be also connected with the strong “dorsal flexion ” 
consequent on the squatting posture. The astragalo-tibial articula- 
tion thus formed rarely occurs in the higher races; but parallel 
modifications of both the upper and lower ends of the tibia occur 
in the Anthropoids and among the lower Apes (Arthur 
Thomson). 
Until approximately the seventh month of fcetal life, the 
tibial malleolus is larger than the fibular, projecting downwards 
farther than the latter. In the seventh month the two appear 
about equal, and then the fibular malleolus begins to take the lead. 
These phases of development are accompanied by corresponding 
modifications of the astragalus (Gegenbaur). 
_ That the earlier condition of these bones is the inherited one 
seems probable from comparison of those of the Lemuroidea, Apes, 
and lower human races. Fig. 59 illustrates the manner in 
which the externalor fibular malleolus (c.f) gradually,in adaptation 
1 [The human fibula has been stated by Leboucq, Bernays, and others, to be 
during early development in contact with the femur, from which it would appear that 
the loss of connection between the two takes place ontogenetically. Griinbaum, 
examining the parts with extreme care, has lately shown (Jowr. Anat. and Phys., 
vol. xxvi. p. xx.) that this is not the case from the period of primary differentia- 
tion of the parts in cartilage onwards. | 
2 In many lower Mammals it has still further degenerated. 
