484 SURGICAL APPLIED ANATOMY. [Chap, xxi 



in connection with the causation of the lesion, that in 

 flexion and extension the two cartilages move with the 

 tibia upon the femur, but in the rotation movements 

 of the leg one or other disc is fixed, and the tibia 

 rotates beneath them. Dr. Scott Lang has shown 

 that the internal cartilage is very much more fre- 

 quently luxated than is the external. He has pointed 

 out that displacement of the internal disc occurs 

 through violence applied when the leg is rotated out- 

 wards, while displacement of the external cartilage 

 can only occur when the leg is rotated inwards. 

 " Rotation outwards is performed chiefly by the biceps ; 

 consequently, when this movement is taking place, the 

 tibia will be found to be held closely to the external 

 condyle of the femur, and the outer cartilage is thus 

 held firmly in its place between the two bones. But 

 the gap between the internal condyle and the tibia 

 will be increased, and the movable cartilage is apt 

 to slip between the internal condyle and the corre- 

 sponding part of the tibia. When the tibia is rotated 

 inwards the internal cartilage is held in like manner 

 between the bones by the internal rotators, while 

 the gap between the external condyle and the tibia is 

 increased " (Scott Lang). The external cartilage is 

 smaller than the internal, is rounder, is more movable, 

 and possibly on these accounts is less likely to be 

 "nipped " between the bones. 



Gcim-valgiim, or knock-knee. The appearances 

 produced by this affection are familiar. When a person 

 stands erect with the feet together, the tibise are practic- 

 ally vertical, and the femora meet them ata certain angle. 

 The degree of this angle depends, in normal subjects, to 

 a great extent upon the relative width of the pelvis. 

 In genu-valgum, the tibise cease to be vertical in the 

 erect position ; their lower ends deviate more and more 

 from the middle line, until the distance between the 

 two nialleoli becomes considerable when the individual 



