THE KNEE-JOINT. 279 



of motion is not a fixed one. If the joint is examined while in a condition of 



extreme flexion, the posterior part of the articular surfaces 



of the tibia will be found to be in contact with the posterior ^OT~ ^S^^ 



rounded extremities of the condyles of the femur ; and if 



a simple hinge-like movement were to take place, the 



axis, round which the revolving movement of the tibia 



occurs, would be in the back part of the condyle. If the 



leg is now brought forward into a position of semiflexion, 



the upper surface of the tibia will be seen to glide over 



the condyles of the femur, so that the middle part of the 



articular facets are in contact, and the axis of rotation 



must therefore have shifted forward to nearer the centre 



. . r IG. loo. view oi tne 



of the condyles. If the leg is now brought into the posterior surface of the pa- 



, i > .. .,, ,, , ,. ,. , tella, showing diagrammat- 



extended position, a still farther gliding takes place and icaiiy the areas of contact 

 a further shifting forward of the axis of rotation. This 

 is not, however, a simple movement, but is accompanied 

 by a certain amount of rotation outward round a vertical axis drawn through the 

 centre of the head of the tibia. This rotation is due to the greater length of the 

 internal condyle, and to the fact that the anterior portion of its articular surface 

 is inclined obliquely outward. In consequence of this it will be seen that toward 

 the close of the movement of extension that is to say, just before complete 

 extension is effected the tibia glides obliquely upward and outward over this 

 oblique surface on the inner condyle, and the leg is therefore necessarily rotated 

 outward. In flexion of the joint the converse of these movements takes place: 

 the tibia glides backward round the end of the femur, and at the com- 

 mencement of the movement the tibia is directed downward and inward along 

 the oblique curve of the inner condyle, thus causing an inward rotation to 

 the leg. 



During flexion and extension the patella moves on the lower end of the femur, 

 but this movement is not a simple gliding one ; for if the articular surface of this 

 bone is examined, it will be found to present on each side of the central vertical 

 ridge two less marked transverse ridges, which divide the surface, except a small 

 portion along the inner border, which is cut off by a slight vertical ridge into 

 six facets (see Fig. 188), and therefore does not present a uniform curved sur- 

 face, as would be the case if a simple gliding movement took place. These six 

 facets three on each side of the median vertical ridge correspond to and denote 

 the parts of the bone respectively in contact with the condyles of the femur during 

 flexion, semiflexion, and extension. In flexion only the upper facets on the patella 

 are in contact with the condyles of the femur ; the lower two-thirds of the bone 

 rests upon the mass of. fat which occupies the space between the femur and tibia. 

 In the semiflexed position of the joint the middle facets on the patella rest upon 

 the most prominent portion of the condyles, and thus afford greater leverage to 

 the Quadriceps by increasing its distance from the centre of motion. In complete 

 extension the patella is drawn up, so that only the lower facets are in contact with 

 the articular surfaces of the condyles. The narrow strip along the inner border 

 is in contact with the outer aspect of the internal condyle when the leg is fully flexed 

 at the knee-joint. As in the elbow, so it is in the knee the axis of rotation in 

 flexion and extension is not precisely at right angles to the axis of the bone, but 

 during flexion there is a certain amount of alteration of plane : so that, whereas in 

 flexion the femur and tibia are in the same plane, in extension the one bone forms 

 an angle of about ten degrees with the other. There is, however, this difference 

 between the two extremities : that in the upper, during extension, the humeri are 

 parallel and the bones of the forearm diverge; in the lower, the femora converge 

 below and the tibia are parallel. 



In addition to the slight rotation during flexion and extension, the tibia enjoys 

 an independent rotation on the condyles of the femur in certain positions of the 

 joint. This movement takes place between the interarticular fibro-cartilages and 



