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observations be absolutely correct, but on the contrary describes a 
helix, consisting of at least two parts, an upper and a lower. This 
observation led the author to the detection of two screw combinations 
in the knee-joint ; and by a careful study of the anatomical relations 
of the elements of the articulation he came to the following conclu- 
sions, 
1. The knee-joint consists essentially of two conical screw com- 
binations. 
2. One of these screw combinations forms the anterior, the other 
the posterior part of the joint. 
3. The axes of these screw combinations, instead of being at right 
angles to the so-called plane of flexion and extension, as in the ankle 
and elbow joints, are parallel, or nearly so, to the axis of the limb, 
the vertices of the fundamental cones being directed upwards. 
4. The femoral condyles form the concave ; the tibial condyles, 
intercondyloid spine, and crucial ligaments, the convex elements of 
each screw combination. 
5. Each of these screw combinations is double-threaded ; the 
breadth and obliquity of the threads rapidly increasing from vertex 
to base. 
6. A comparatively limited extent of the convex element of each 
combination is retained, so that the larger extent of the concave ele- 
ment employed moves on the former by a combination of gliding in 
the direction of the screw, and of rolling. 
7. The gliding in the direction of the screw is due partly to the 
screwed configuration of the opposite cartilaginous surfaces ; partly 
to the peculiar mode of attachment of the crucial ligaments. 
8. In consequence of the peculiar attachments of the successive 
fasciculi of the crucial ligaments, these fasciculi, after having in suc- 
cession co-operated in producing the gliding movement in the direc- 
tion of the screw, bend over, and thus permit the rolling move- 
ment. 
9. The path described by any point in the thigh or leg during 
flexion or extension of the knee-joint is a helix, produced by the 
movements of the two screw combinations in succession : but modified 
by the rolling. 
10. The anterior screw combination is left-handed in the right 
knee, and right-handed in the left. 
