106 



THE HUMAN MOTOR 



vature ( l ) form an angle (i " inferior " to the angle a given by 

 the radii of the major curvature (fig. 115). 



The name of a " saddle- joint " is given to the joint where the 

 surface of the fixed bone is analogous to the back of a horse's 

 saddle, the surface of the mobile bone occupying a direction 

 perpendicular thereto. The only clear example is that of the 

 trapezium / (fig. 116) jointed to the metacarpal bone of the 

 thumb ; one saddle covers the other. The movement differs 

 very little from that of the oval joint. 



(iii) Three degrees of liberty are observed in the " ball and 

 socket," or three axes joints. Centred one on the other, the 

 joint-head or " ball " and the spheric cavity or cap are such that 

 the former fits into the latter so that the head of the mobile 

 bone touches the sides of the cavity, and the greater the radius of 

 the joint head is than that of the cavity, the greater will be the 

 freedom of movement. 



t 



FIG. 



Joint of the trapezium to the metacarpus (saddle-joint). 



(*) The curvature is small when it forms part of a circumference with a 

 large radius and conversely ; the radius of the curve is therefore the 

 inverse of the curvature. 



