MOTIONS OF THE FORE ARM. 207 



natural, the axis of the cone, instead of being directly outwards, 

 is somewhat forwards. 



By rotation, is meant the revolving of the os humeri upon 

 itself. The centre of this movement is not the axis of the bone 5 

 but is removed to one side of it, by the lateral projection of the 

 head. The neck, however, is too short and thick to permit 

 any great extent to this motion; it, accordingly, is limited in 

 such a way as never to amount to luxation. Its greatest ex- 

 tent, in most persons, does not exceed the describing of half a 

 circle, which may be ascertained by applying a finger upon 

 the internal condyle of the os humeri. By it the capsular liga- 

 ment is rendered, alternately, loose and tense on its front and 

 back parts. Bichat observes, that in the anchylosis of the 

 elbow joint, this motion, by habit, is much augmented, so as to 

 supply the want of rotation of the head of the radius upon the 

 ulna. The scapula and the clavicle do not vary their position 

 in rotation. 



SECT. IX. OF THE MOTIONS OF THE FORE ARM. 



There are two kinds of motion in the fore arm. In the one, 

 the fore arm is flexed, or extended upon the arm, and in the 

 other, the radius only changes its position in regard to the 

 ulna. 



1. The ulna is the essential agent of the first, in consequence 

 of its manner of articulation with the os humeri ; the radius is 

 only accessory, and is drawn by the ulna into a participation in 

 its motions. These two bones, it will be recollected, are dis- 

 posed of in an inverse manner, the larger part of the ulna being 

 above, while the larger part of the radius is below. This ar- 

 rangement causes the ulna to present the principal articular 

 surface for union with the os humeri, while the radius affords 

 the principal surface to the carpus; it also gives to the whole 

 fore arm a great uniformity in its transverse diameter. The 

 fore arm executes, upon the arm, flexion, extension, and lateral 

 inclination. 



Where the flexion is complete, the coronoid process is re- 

 ceived into its cavity, on the front of the os humeri; and the ole- 



