THE CHIMPANZEES AND ORANGS. 19 



ri6/«.— 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 fiat, with slight inequalities, 

 of an irregular oblong form, relatively broader from side to side than in Man (tig. 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 crucialligament near the 

 spine (6). The tuberosity (fig. I, d) overhanging the fibula is more developed than in 

 Man ; that (/) for the rotular ligament is rather less prominent. The head rapidly 

 contracts to the shaft. The straight line of the inner border (t) bends very little at the 

 expansion (a) for the inner malleolus (»), 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 (/) 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 (PI. 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 (6) 



d2 



