240 MECHANISM OF THE KNEE-JOINT. 



This curve, being more leisurely examined by means of a 

 plumb-line suspended between it and the eye, or through a 

 cross-threaded frame, or with a telescope with parallel wires in 

 the focus of its eye-piece, was found to be more rapid in its 

 curvature at the upper and lower end, while in the inter- 

 mediate portion of its extent it lay more in the direction of 

 the plumb-line. 



It was now observed, on examining the joint itself, not 

 only that the condyles of the femur roll backwards and for- 

 wards in curved paths — a fact which has hitberto escaped 

 notice — but that this curvature or rotation is as strongly 

 marked at the flexion as at the extension extremity of the 

 movement. 



By fixing the joint in the horizontal plane, and setting up 

 a series of rods from that plane to the point of the indicator 

 fixed in the femur, at equal intervals in its course, I have pro- 

 cured a system of co-ordinates which enable me by the eye to 

 trace more carefully the course of the curves on the three 

 planes on which it is projected. By this rough method, 

 altogether insufficient for precise results, the eye is nevertheless 

 enabled to detect — 1. That the line is a helix, with variable 

 curvature, with a more rapid sweep at its upper and lower 

 portions than in the intermediate distance, the upper sweep 

 being the larger of the two ; 2. That there are two breaks in 

 its course, the first near its upper end at the commencement 

 of the great sweep, and most perceptible in the mesial plane ; 

 the second is situated about the junction of the lower and 

 middle third of the line, and consists of an angular bend, with 

 a more rapid curvature on each side, observable in the mesial 

 and transverse planes. 



These peculiarities in the curve induced me to re-examine 

 the curvatures of the femoral condyles, and I found that the upper 

 break corresponded to the grooves between the patellar and 

 condyloid surfaces, and also recognised what has also hitherto 



