CHAP. V.] JOINTS. 49 



and admit of only a limited amount of gliding movement, 

 as in most of the articulations of the wrist and ankle, and in the 

 joints between the articular processes of the vertebrae. 



(2) Hinge joint. The articular surfaces are of such shape 

 as to permit of movement, to and fro, in one plane only, like a 



'door on its hinges. These movements are called flexion and 

 extension, and may be seen in the articulation of the arm with 

 the forearm, in the ankle joint, and in the articulations of the 

 phalanges. 



(3) Ball and socket joint. In this form of joint a more or 

 less rounded head is received into a cup-like cavity, as the head 

 of the femur into the acetabulum, and the head of the humerus 

 into the glenoid cavity of the scapula. Movement can take 

 place freely in any direction, but the shallower the cup, the 

 greater the extent of motion. 



(4) Pivot joints. In this form, one bone rotates around 

 another which remains stationary, as in the articulation of the 

 atlas with the axis, and in the articulation of the ulna and 

 radius. In the articulation of the ulna and radius, the ulna 

 remains stationary and the radius rotates freely around its 

 upper end. The hand is joined to the lower end of the radius, 

 and the radius, in rotating, carries the hand with it; thus, the 

 palm of the hand is alternately turned upwards and down- 

 wards. When the palm is turned upwards, the attitude is 

 called supination ; when downwards, pronation. 



The different kinds of movement of which bones thus con- 

 nected are capable are, flexion and extension ; abduction and 

 adduction ; rotation and circumduction. 



A limb is flexed, when it is bent; extended, when it is 

 straightened out. It is abducted, when it is drawn away from 

 the middle line ; adducted, when it is brought to the middle 

 line. It is rotated, when it is made to turn on its own axis; 

 circumducted, when it is made to describe a conical surface, by 

 rotation around an imaginary axis. No part of the body is 

 capable of perfect rotation, as a wheel, for the simple reason that 

 sucli motion would necessarily tear asunder all the vessels, 

 nerves, muscles, etc., which unite it with other parts. 



As the synovial membranes are intimately connected with 

 the joints, it may be well to give a brief description of 

 them here. 



