304 



HUMAN ANATOMY. 



a motionless ulna is not what actually occurs in life. Let the reader grasp lightly 

 his right wrist with his left thumb and forefinger, so that they nearly meet at the 

 styloid process of the radius, and, pressing the right elbow to the side for steadiness, 



Fig. 319. 



Internal 

 condyle 



Posterior part 

 of capsule 



Olecranon 



Interosseous membrane 



Ulna Tendon of biceps 



Superficial layer of 

 inter, lat. ligament 



Deeper layer of internal 

 lateral ligament 



Right elbow-joint, inner aspect. 



Fig. 320. 



pronate the right arm. The lower end of the radius will occupy the place at the 

 base of the left thumb previously occupied by the ulna, which will have travelled 

 outward along the left forefinger. It is very doubtful whether in this experiment all 

 motion at the shoulder is eliminated ; nevertheless, the ulna undoubtedly changes its 

 place, and with equal certainty it does not " rotate." To prove this, let the arm of 



a subject be held in a vice above the elbow, which should be 

 seniifle.xed, and, the forearm being supine, let a long pin 

 pointing outward be fixed into the outer side of the radius 

 above the wrist, and another pointing inward into a corre- 

 sponding point of the ulna. On pronating the hand, the pin 

 in the radius will describe a large curve and that in the ulna 

 will make no evident movement. On close inspection, aided 

 by placing some object close to the head of the pin in the 

 ulna, it will appear that, though the bone has not rotated, 

 the pin-head has changed its place : it has moved down- 

 ward and outward. Jf the hand be now disarticulated, aild 

 two pins bearing brushes dipped in paint be placed in the 

 end of the head of the ulna and in the lower surface of the 

 radius, pointing downward so as to continue the line of the 

 shafts of these bones, on twisting the forearm, each of these 

 brushes will describe a curve on a sheet of paper held against 

 them ; that in the radius making a large curve upward and 

 inward, and the ulnar pin a small one downward and out- 

 ward. The relative size of these curves inay be \'aried 

 greatly by the operator. What has occurred is this : besides 

 the rotation of the radius, there has been a lateral movement between the ulna and 

 humerus combined with a slight extension. This movement is less when the arm is 

 nearly straight than when flexed, for in the latter position the lateral parts of the 



Diagram showing the 

 equal angles of the long 

 axes of the. bones with the 

 axis of the joint. 



