76 



THE HUMAN BODY 



wards and the thumb outwards, the 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 dis- 

 cerned 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 

 got to its inner side. The relative position of the bones when the 

 palm is upwards is shown at A in Fig. 37, and when the palm is 



down at B. The former position is 

 known as sypination; the latter as 

 pronation. The elbow end of the 

 humerus (Fig. 37) bears a large artic- 

 ular surface: on the inner two-thirds 

 & of this, the ulna fits, and the ridges 

 and grooves of both bones interlocking 

 form a hinge-joint, allowing only of 

 bending or straightening the forearm 

 on the arm. The radius fits on the 

 rounded outer third, 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. 

 T-,- 8 _, Gliding Joints. These permit as a 



FIG. 37. A, arm in supina- e 7 



tion; B, arm in pronation. H, rule but little movement: examples 



are found between the" closely packed 



bones of the tarsus and carpus (Figs. 31 and 32), 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 inflammation and swelling 

 which make not only the recognition of the injury but, after- 

 diagnosis, the replacement of the bone, or the reduction of the dis- 

 location, 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 be attempted with safety 

 by one who knows the form of the bones, and possesses sufficient 



