TRANSACTIONS OF SECTION H. 777 



standing, and may be called appropriately the extensor area. When the femur ia 

 abducted it passes within the acetabulum. The head of the femur in man is not so 

 sharply differentiated from the neck as in the Anthropoid apes, especially in the 

 region of the extensor articular area. 



Both man and apes possess at the lower end of the femur a trochlear or pulley- 

 like surface in front for the patella, and two condyles for the tibia. In the apes 

 the trochlea is shallow, and the concave curve from side to side is a segment of an 

 approximate circle, with a large radius. In man the trochlea is much deeper, and 

 the inner and outer parts of the curve deviate considerably from a circle, and 

 are not symmetrical ; the outer part is wider and extends higher on the front of 

 the bone than the inner part, whUst the direction of the curve changes towards 

 the edges of the trochlea. 



In the apes the articular surface of the inner condyle is very markedly 

 larger than that of the outer condyle, both in breadth and in the extent of its 

 backward curve, which winds upwards on the posterior part of the condyle, so 

 that the articular surface is continued on to its upper aspect. The curve of the 

 outer condyle is much sharper, and the condyle does not project so far backwards ; 

 its articular surface is not prolonged so high on the back of the bone. In the 

 apes, therefore, the inner is the more important condyle in the construction of the 

 knee joint, and the marked extension of its articular area backwards and upwards 

 is associated with the position and movements of the knee in flexion. In the ape 

 the thigh is more rotated outwards than in man, and the inner condyle is directed 

 to the front of the limb. 



In man there is not nearly the same dLsproportion in the size of the two con- 

 dyles as in the apes. I have occasionally seen in man the articular area of the 

 inner broader than that of the outer condyle, but more usually the outer is appre- 

 ciably the wider. The backward curve of the outer condyle is also prolonged 

 somewhat higher than that of the inner, and thus the condition of the two con- 

 dyles is the reverse of that found in the ape. It should, however, be stated, as has 

 been shown by Dr. Havelock Charles,' that in persons who habitually rest in the 

 squatting position, an upward extension of the articular area of the inner condyle 

 exists, which is associated with the acute flexion of the knee whilst squatting. 

 In man, the outer condyle, when seen in profile, is, as compared with the inner, 

 more elongated antero-posteriorly than in the Gorilla. The approximate equality 

 in the size of the two condyles in man is, without doubt, associated with the ex- 

 tension of the knee joint in the erect attitude, and with tlie more equable distribu- 

 tion of the weight of the body downwards on the head of the tibia. In the ape 

 the intercondylar fossa, in relation to the size of the bones, is wider in front than 

 in man ; but it is wider behind in man than in the ape, for in the latter the inner 

 condyle inclines nearer to the outer condyle than in man. 



In man, when the knee joint is extended, the tibia is slightly rotated outwards 

 on the femoral condyles, and the joint is fixed, partly by the tension of the lateral 

 and posterior ligaments and the anterior crucial ligament, and partly by the gene- 

 ral tension of the muscles and fascife around the joint. So long as these structures 

 remain tense, the joint cannot be bent, and no lateral movement, or rotation, ia 

 permitted. The fixation of the joint is of fundamental importance in the act of 

 standing. Free rotation of the human knee can only take place when the joint is 

 acutely bent. 



In apes, the joint cannot be fully extended ; its natural position, when the 

 animal is standing, is partial flexion, and in this position a limited rotation is per- 

 mitted, which can be greatly increased when the joint is more completely bent. 

 Id rotating the leg on the thigh the inner condyle is apparently the pivot. The 

 rotation facilitates the use of the foot as an organ of prehension, and assists the 

 ape to turn the sole inwards and forwards when holding an object. These move- 

 ments produce results, which approximate to those occasioned by pronation and 

 supination of the radius on the ulna, in the movements of the forearm and hand. 



In the Anthropoid apes, the head of the tibia slopes very decidedly backwards 

 at the upper end of the shaft, so that its axis forms an angle with that of the shaft, 



' J(mrnal of Anatomy and Physiology, vol. xxviii. 



