THE CHIMPANZEES AND ORANGS. 19 
Tibia.—Plate VIII. 
In Man the tibia (fig. 9), after the femur, is the longest bone of the skeleton ; but in 
the Gorilla (fig. 1) the tibia is the shortest of all the long bones of the limbs, being 
barely two-thirds the length of the humerus. It is nearly one-fifth shorter than the 
Human tibia, but is of equal thickness in the shaft, and of greater thickness at the 
upper end. 
The upper surface of the head of the bone (fig. 2) is nearly flat, with slight inequalities, 
of an irregular oblong form, relatively broader from side to side than in Man (fig. 10) ; 
it is divided into two articular and two non-articular facets. The inner articular facet 
(fig. 2, n) presents an almost semicircular form with the nearly straight border outward ; 
the very slight concavity of the surface is due chiefly to the rising of the middle part of 
the straight border into a low eminence (b), giving attachment to the extremity of the 
semilunar cartilage of that side. The outer facet (e) is rather more elevated along its 
oblique inner border, which rises to the tuberosity (b') for the attachment of its semi- 
lunar cartilage ; this surface occupies a smaller proportion of the head of the bone than 
in Man. The anterior non-articular tract is of a triangular form with a slight anterior 
eminence (c), and a depression for the attachment of the crucial ligament near the 
spine (6). ‘The tuberosity (fig. 1, d) overhanging tie fibula is more developed than in 
Man ; that (f) for the rotular ligament is rather less prominent. The head rapidly 
contracts to the shaft. The straight line of the inner border (¢) bends very little at the 
expansion (a’) for the inner malleolus (i), the concavity there being much less than in 
Man: the outer border differs still more by its concave sweep, leaving the wide inter- 
osseous space between the tibia and fibula. Below and external to the rotular tube- 
rosity (f) is an extensive rough shallow depression for the attachment of the sartorius, 
semitendinosus, and gracilis muscles. The ‘ crest’ of the tibia curves towards the inner 
and fore part of the lower end of the shaft with its concavity outward, giving a greater 
proportional extent of origin to the ‘ tibialis anticus’ than in Man. The anterior contour 
of the tibia is principally convex, instead of being concave; the posterior contour is 
slightly concave. The oblique line for the origin of the ‘soleus’ is strongly marked at 
the upper and back part of the shaft; a little below this is the orifice of the canal for 
the medullary artery leading down to the cavity of the bone, as in Man. The inner 
malleolus is more angular in form and projects rather more downward, but is less 
extended outward, than in Man. The quadrilateral articular surface on the lower end 
of the bone (fig. 3) is less concave from before backward than in Man, and becomes 
slightly convex anteriorly, allowing a freer movement of the foot upon the leg. 
The tibia of the Chimpanzee (Pl. VIII. fig. 5) deviates further than that of the 
Gorilla from the Human type, in the proportionally more expanded extremities and 
more slender shaft. The inner articular facet of the proximal end (fig. 6, n) is less 
defined and relatively smaller than in the Gorilla or Man ; the tubercles of the spine (0) 
p 2 
