HINGE- AND BALL-AND-SOCKET JOINTS; DISLOCATIONS. 457 



articulations, the hinge-joint, and the ball-and-socket joint. 

 In the first of these, the surfaces of the bones are so formed 

 that the movement, though free as regards its extent, is very 

 limited in its direction; being in fact restricted to a back- 

 ward and forward action in the same line, just like that given 

 by a common hinge. In the second, the end of one bone is 

 formed into a rounded head or ball, and this is received into 

 a corresponding socket or cup in the other, the edge of which 

 is usually deepened by cartilage ; in this manner the bone 

 which carries the ball is enabled to move upon the other in 

 any direction, unless restrained by external checks. Of the 

 hinge-joint we have examples in the elbow, the knee, and the 

 joints of the fingers and toes. Of the perfect ball-and-socket 

 joint we have in Man only two examples, the shoulder, and 

 the hip. In the former the socket is much shallower than in 

 the latter, and the motions of the arm are consequently more 

 extensive than those of the thigh : both, however, are un- 

 checked in regard to their range and direction, except when 

 the limb is brought against the body or against its fellow. 

 The wrist and the ankle-joint are of an intermediate cha- 

 racter; the former more resembling the ball-and-socket, and 

 the latter the hinge-joint. 



604. All these joints are more or less subject to dislocation, 

 by violence of different kinds. This takes place by the 

 slipping-away from each other of the two surfaces, which 

 ought to be in contact. Thus the head of the humerus (or 

 arm-bone) may slip over the edge of its socket, so as to lie 

 entirely on the outside of it ; and this, in consequence of the 

 shallowness of the cup, happens not unfrequently. The head 

 of the thigh-bone, also, may slip out of its socket ; but this 

 accident is more rare, on account of the deepness of its cup. 

 The elbow and knee-joints, as also those of the wrists, ankle, 

 fingers, and toes, may be dislocated by the slipping of one 

 surface on the other, either forwards or backwards, to one side 

 or to the other. One of the most common dislocations is that 

 of the thumb, the lowest articulation of which has rather the 

 character of the ball-and-socket (with a very shallow cup), 

 than of the hinge-joint. But in proportion to the liability of 

 any joint to dislocation, is usually the ease with which it may 

 be brought into place again. 



605. Of the attachments of muscles to the skeleton, one is 



