THE HUMAN STRUCTURE 



105 



Fir, 113 



- L -\D 



(ii) Movement with two degrees of liberty is shown clearly 

 M in '\oval joints " with two very unequal axes 

 (ellipsoid joints) or nearly equal axes (spheroid 

 joints) ; the larger axis being perpendicular to 

 the direction of the limb, that is to say, trans- 

 verse, whereas, if the surfaces fit tightly, the only 

 movement possible will be round the larger axis 

 AB (fig. 114) limited by the contact of the moving 

 bone with the edges of the cavity of the joint. 



Such is the case in the joint of the forearm to the wrist (radio 



carpal) which^would only have one degree of liberty, but that, as 



a rule, movement is equally possible round 



the small axis CD, and the moving bone 



can turn on itself ; by which rotation, or 



torsion with flexion and extension, is 



produced. It must be added that the 



oval or ellipse has two different curves, 



that is to say, two centres of curvature, 



so that the joint head, in its double 



movement, does not turn round the same 



point, for that point changes position. 



All the intermediate axes can be like 



AB and CD axes of rotation ; but it 



can be shown that the movement takes 



place round the axis, which causes the Oval joint. 



" minimum " action on the part of the cartilages f 1 ) 



This was noted by Listing in the movement of the eye-ball. 



Listing's law establishes that 

 the movement of the eyes 

 entails a very feeble effort by 

 the oculomotor muscles of the 

 eyes. On this subject, we may 

 recall that the moment of 

 inertia of an ellipsoid is a mini- 

 mum, when the rotation is round 

 the major axis, and the angular 

 displacement is less on that 

 axis than on the minor axis ; 

 for the same displacement mn, 

 tt . the two radii of the minor cur- 



( l ) Fischer, Ueber Gelenke von zwei graden der Fretheit (Arch. f. Anat.), 

 1897, Suppl., p. 242 ; Otto Fischer, Zuv Kinematik des Listingschen Gesetzes 

 (Abhandl. d. Math. Phys. Klasse d. Konigl. Sachs. Gesell, d. Wtss., 1909, 

 vol. xxxi.) 



