210 PATELLA. 



any bone of its bulk Is so strong. But, notwithstanding this 

 strength, it is sometimes broken by the violent straining effort 

 of the muscles. 



The principal motions of the knee joint are flexion and 

 extension. In the former of these, the leg may be brought to 

 a very acute angle with the thigh, by the condyles of the thigh 

 bones being round, and made smooth far backwards. In 

 performing this, the patella is pulled down by the tibia. When 

 the leg is to be extended, the patella is drawn upwards, conse- 

 quently, the tibia forwards, by the extensor muscles; which, by 

 means of the protuberant joint, and of this thick bone with its 

 ligament, have the chord, with which they act, fixed to the 

 tibia at a considerable angle, and act, on that account, with 

 advantage; but they are restrained from pulling the leg farther 

 than to a straight line with the thigh, by the posterior part of 

 the cross ligament, that the body might be supported by a firm 

 perpendicular column : for, at this time, the thigh and leg are 

 as little movable in a rotary way, or to either side, as if they 

 were one continued bone. But, when the joint is a little bent, 

 the rotula is not tightly braced, and the posterior ligament is 

 relaxed ; therefore, this bone may be moved a little to either 

 side, or with a small rotation in the superficial cavities of the 

 tibia ; which is done by the motion of the external cavity 

 backwards and forwards, the internal serving as a sort of axis. 

 Seeing, then, one part of the cross ligament is situated perpen- 

 dicularly, and the posterior part is stretched obliquely from the 

 internal condyle of the thigh outwards, that posterior part of the 

 cross ligament prevents the leg from being turned much inwards ; 

 but it could not hinder it from turning outwards almost round, 

 were not that motion confined by the lateral ligaments of this 

 joint, which can yield little. 



This rotation of the leg outwards is of great advantage to us 

 in crossing our legs, and turning our feet outwards, on several 

 necessary occasions ; though it is necessary that this motion 

 should not be very large, to prevent frequent luxations here. 

 While all these motions are performing, the part of the tibia 

 that moves immediately on the condyles is that which is within 



