MOTIONS OF THE FORE ARM. 209 



the muscles which flex the fore arm are kept so much in the 

 line in which they contract, or are so little removed from the 

 axis of their own motion, that they contribute but little to sus- 

 tain the fore arm in situ; the weight is, therefore, actually sus- 

 tained by the ligaments in front of the articulation. But they 

 being pressed and drawn in the manner mentioned, such great 

 pain and weariness are produced as to render a continued sus- 

 pension of the weight insupportable, the experimenter is, there- 

 fore, in a short time, under the necessity either of casting off 

 the weight or of giving such a degree of flexion to the fore arm 

 as will allow the muscles to contract more advantageously. 



Besides flexion and extension, the ulna has a sort of rocking 

 motion when the fore arm is only half bent; but when the lat- 

 ter is at either extreme of the former positions, this motion is 

 imperceptible, owing to the nature of the articular surfaces and 

 the resistance of the ligaments. 



2. In the rotations of the radius upon the ulna, the latter is 

 almost motionless, excepting the case specified in the last para^ 

 graph. The position of the radius on a plane somewhat ante- 

 rior to the ulna, its small cylindrical upper extremity, and its 

 broad lower one, all concur in facilitating rotations forwards 

 and backwards. It is owing to the hand following these mo- 

 tions that the first is expressed by the term pronation, in which 

 the palm of the hand is directed downwards ; and the second, 

 supination, in which the palm is upwards and the back of the 

 hand downwards. 



Pronation is the most common, and, consequently, the easiest 

 position to the fore arm, when not carried to an extreme: it is 

 adopted involuntarily, simply by the action of the ligaments and 

 the particular shape of the articulating surfaces of the bones. 

 It is the posture most generally suited to the examination and 

 grasping of surrounding bodies. In order that it may be ac- 

 complished fully, the superior extremity of the radius rolls on 

 its own axis, in the loop formed by the annular ligament and 

 the lesser sigmoid cavity of the ulna; while the lower extremi- 

 ty revolves around the little head of the ulna below. The mid- 

 dle part of the radius crosses that of the ulna, and the interos- 

 seous space is diminished. An excess of this motion will pro* 

 duce luxation either above or below, but more easily at tha 



18* 



