708 MAMMALIAN ANATOMY 



and within downward, backward, and outward, and therefore when 

 the bone is held vertically it faces upward, backward, and outward. 

 Its outer border is the longest, and is slightly arcuate. Its anterior 

 border is the shortest ; it is straight and directed inward and back- 

 ward. Just in front of the antero-internal angle is a pit, more or less 

 well marked, into which is inserted one end of the internal semilunar 

 cartilage. The inner border is arcuate, and prominent, especially 

 behind the middle, where a decided swelling forms one-half of the 

 so-called spine of the tibia. On the inner surface of this swelling, 

 and in the groove itself, is inserted the other end of the internal semi- 

 lunar cartilage. The posterior border is slightly arcuate and rounded ; 

 it forms the overhanging upper edge of the posterior surface of the 

 head. The postero-external angle is prominent and is directed out- 

 ward, backward, and downward ; it bears a more flattened surface, 

 which articulates with the sesamoid in the tendon of the popliteus 

 muscle. In the groove and on the inner border, behind the insertion 

 of the internal semiluuar cartilage, is inserted one end of the external 

 semilunar cartilage. 



The inner articular surface is narrower transversely than the 

 outer articular surface. Its outline is oval, and the long axis is directed 

 antero-posteriorly. The lateral and median margins are slightly arcu- 

 ate ; the latter is interrupted by a swelling which forms the other half 

 of the tibial spine and affords attachment to the outer end of the ex- 

 ternal semilunar cartilage. The articular surface does not cover the 

 entire inner posterior half of the upper surface of the bone, but leaves 

 a strip along the posterior margin for the insertion of the posterior 

 crucial ligament. The tibial end of the anterior crucial ligament is 

 attached to the median margin and to the inner articular surface itself 

 just in front of the insertion of the external semilunar cartilage. The 

 surface is slightly concave from side to side and convex from before 

 backward. 



The groove which separates the two articular surfaces is narrowed 

 in the middle by the approach of the two prominences which together 

 are called the spine. These prominences are not directly opposite 

 each other ; the external lies somewhat posterior to the internal. 



The articular surfaces are covered by the semilunar fibro-cartilages, 

 which form two well-defined cups in which the condyles of the femur 

 move backward and forward. 



