240 



ANIMAL MECHANICS. 



the distal end of the radius can be swung around the comparatively 

 immovable ulna. The head of the radius remaining stationary while 

 the lower end moves in a circle round the distal end of the ulna, the 

 shaft of the radius moves on the surface of a cone, of which the axis 

 passes through the centre of the capitulum radii above and through the 

 styloid process of the ulna below. This is the mechanism of the 

 movements termed pronation and supination. 



The movements at the knee-joint were carefully studied by the 

 Webers. 1 According to these observations, movements occur in a sagittal 

 plane (flexion and extension), through an angle of about 150. The axis 

 of rotation is, however, not a fixed axis, but shifts during the acts of 

 extension and flexion. The articular surface of the tibia is almost 

 horizontal, and upon it the condyles of the femur roll like the wheels 

 of a carriage, rolling forwards during extension, and rolling back- 

 wards during flexion. But 

 this rolling is not a simple 

 rolling in one plane, but it 

 is combined with a rotation 

 upon a vertical axis ; the con- 

 dyles moving in relation to 

 the surface of the tibia, like 

 the fore-wheels of a carriage 

 as it turns a corner move on 

 the ground. 



Meyer, 2 like the Webers, 

 in addition to that rotation 

 which is bound up, as it 

 were, with the act of flexion, 

 recognises some rotation upon 

 a vertical axis, which can be 

 produced in the bent but 

 not in the extended limb. 

 Sappey 3 described slight 

 lateral movements of the 

 joint. 



Braune and Fischer, 4 who have investigated in the living subject the 

 movements of the knee in their relation to co-ordinate planes, agree 

 in the main with the conclusions of Weber. Their most important 

 additions appear to be the following. In respect of the rotation which 

 accompanies and is bound up with the act of flexion, they find that, 

 commencing with the extended position, and ending with that of 

 complete flexion, there is at first, for about 20, a rotation inwards. 

 This amounts in all to 6. Then follows a slighter rotation outwards, 

 but as this lasts longer than the inward rotation that preceded it, it 

 finally reaches 6 in the opposite direction, one effect counterbalancing 

 the other. 



Saddle joints and ellipsoidal joints. We may conceive the saddle- 

 shaped surface as generated in the following manner (Fig. 128). 



Take the arc ab of a small circle, the centre of which is at c, 

 and rotate it upon the axis xx, and it will produce the figure abb'af. 



1 "Gehwerkzeuge," S. 170. 



2 "Statik und Mechanik des menschlichen Knochengeriistes," Leipzig, 1873, S. 357. 



3 " Aiiatomie descriptive." 4 "Die Bewegimgen des Kniegelenks," Leipzig, 1891. 



FIG. 128. To show the geometrical construction 

 of a saddle-shaped joint. 



