SPECIAL PHYSIOLOGY OF THE MUSCLES. 203 



(b) The straight line docs not pass through the centre 

 of the circle. 



(a) It lies on the concave side of the arc : oval plane, 

 oval joint . 



The oval joint has two axes, of which one coincides with 

 the rotation axis of the plane of rotation ; the other, passing 

 through the centre of the arc, is perpendicular to the first 

 axis. Example: radio-carpal joint. 



(#) The straight line lies on the convex side of the arc: 

 saddle-joint. 



The saddle-joint has two axes which are analogous to 

 those of the oval joint. Example : joint between the trape- 

 zium and the metacarpus pollicis. 



2. The curve has any form except that of a circle, and 

 the straight line may have any position. 



Such joints are called hinge-joints; they have one axis of 

 rotation which coincides with the axis of the plane of rota- 

 tion. If one of the two bones forming a hinge-joint is 

 imagined to be fixed, a given point of the second bone, 

 on moving, describes a circle. Example: joints of the 

 phalanges. 



As special cases we must also mention : 



1 . The screwed- surface joint, a joint with one axis and in 

 which a given point of the supposed movable bone describes 

 a spiral line instead of a circle. In the screwed -surface joint 

 the bones, during turning, slide over each other in opposite 

 directions but move in the direction of the axis. Example: 

 the elbow. 



2. The spiral joint. The plane of rotation of the spiral 

 joint may be conceived of as follows: The curve which by 

 its rotation describes the plane of rotation approaches during 

 this movement nearer to the straight line. A given point 

 in the curve, therefore, does not describe a circle but a 

 spiral. Hence a given point of the imaginary movable bone 

 describes not a circle but a spiral. Example: the knee. 



Mechanisms by which the joints arc Jicld together. Besides 

 the ligaments (ligamenta accessorial of the hinge-joint) and 



