96 



THE HUMAN BODY. 



fr- 



lower end of the radius can be felt on the outer side of the 

 forearm just above the wrist, and if this be done while the hand 

 is turning over, it will be easily discerned that during the 

 movement this end of the radius, carrying the hand with it, 

 travels around the lower end of the ulna so as to get to its 

 inner side. The relative position of the bones when the palm 

 is upwards is shown at A in Fig. 44, and when the palm is 



down at B. The former position 

 is known as supination ; the latter 

 as pronation. The elbow end of 

 the humerus (Fig. 39) bears a 

 large articular surface: on the 

 inner two thirds of this, Tr, the 

 ulna fits, and the ridges and 

 grooves of both bones interlock- 

 ing form a hinge-joint, allowing 

 only of bending or straightening 

 the forearm on the arm. The 

 radius fits on the rounded outer 

 third, Cpl, and forms there a ball- 

 and-socket joint at which the 

 movement takes place when the 

 hand is turned from the supine 

 to the prone position ; the ulna 

 forming a fixed bar around which 

 the lower end. of the radius is 

 moved. 



Gliding Joints. These per- 

 mit as a rule but little movement: 

 examples are found between the closely packed bones of the 

 tarsus (Fig. 38) and carpus, which slide a little over one 

 another when subjected to pressure. 



Hygiene of the Joints. When a bone is displaced or 

 dislocated the ligaments around the joint are more or less 

 torn and other soft parts injured. This soon leads to inflam- 

 mation and swelling which make not only the recognition of 

 the injury but, after diagnosis, the replacement of the bone, 

 or the reduction of the dislocation, difficult. Moreover the 

 muscles attached to it constantly pull on the displaced bone 

 and drag it still farther out of place; so that it is of great 

 importance that a dislocation be reduced as soon as possible. 

 In most cases this can only Ipe attempted with safety by one 



FIG. 44. A, arm in supiuation; B, 

 arm in pronation. H, humerus; R, 

 radius; U, ulna. 



