HINGE- AND BALL-AND-SOCKET JOINTS ;—DISLOCATIONS. 457 
articulations,—tlie hinge- joint, and the ball-and-socket- joint. 
In the first of these, the surfaces of the hones 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 
