MOVEMENTS.] PHYSIOLOGY, 23 



like that of a door on its hinge, and that movement is 

 of such a kind as to double the ulna on the humerus. 



Moreover, if you look a little more carefully you will 

 find that, though you can easily double the ulna on the 

 front of the humerus, and then pull it back again until 

 the two are in a straight line, you cannot bend the 

 ulna on the back of the humerus. On examining the 

 end of the ulna you will find at the back of it a beak- 

 like projection (Fig. 3, also Frontispiece), which when 

 the bones are straightened out locks into the end of 

 the humerus, and so prevents the ulna being bent any 

 further back. This is the reason why you can only bend 

 your arm one way. As you very well know, you can 

 bend your arm so as to touch the top of your shoulder 

 with your fingers, but you can't bend it the other way 

 so as to touch the jack of your shoulder ; you can^t 

 bring it any further back than the straight line. 



14. Well, then, at the elbow the two bones, the 

 humerus and ulna, are so shaped and so fit into each 

 other that the arm may be straightened or bent. In the 

 skeleton the two bones are quite separate, i.e. they have 

 to be fastened together by something, else they would 

 fall apart. Most probably in the skeleton you have been 

 examining they are fastened together by wires or slips 

 of brass. But they would hold together if you took 

 away the wire or brass slips and bound some tape 

 round the two ends, tight enough to keep them 

 to xhing each other, but loose enough to allow them 

 to move on each other. You might easily manage 

 it if you took short slips of tape, or, better still, of 

 india-rubber, and placed them all round the elbow, 

 back, front, and sides, fastening one end of each slip 

 to the humerus and the other to the ulna. If you 



