BONES OF THE FOREARM. 185 



the body, there cannot be any force applied at this place to 

 occasion a luxation, since tlie joint is protected so well 

 above. 



The motions which the arm enjoys by this articulation, are 

 to every side ; and, by the succession of these different motions, 

 a circle may be described. Besides which, the bone performs 

 a small rotation round its own axis ; but, when the axis of the 

 bone is the centre of motion, the movements are very different 

 from those which take place when the axis of its head is the 

 centre; for the axis of the head forms a very large angle with 

 the axis of the body of the bone. Thus, when the arm swings 

 backwards and forwards, the axis of the head is the centre of 

 motion : but when the elbow is bent, and the forearm forms a 

 right angle with the os humeri, the motion which applies the 

 forearm to the thorax, or removes it, is a rotation of this bone 

 on its axis. 



Though the motions of the arm seem to be very extensive, 

 yet the larger share of them depends on the motions of the 

 scapula ; for the surface of the glenoid cavity is directed 

 upwards or downwards, and, to a certain degree, backwards 

 or forwards, to support the head of the os humeri. This is 

 exemplified when we press the hand against a body which is 

 before, or above, or to one side of us. 



The lower end of the os humeri is articulated to the bones of 

 the forearm, and carries them with it in all its motions ; but 

 serves as a base, on which they perform the motions peculiar to 

 themselves; as will shortly be described. 



The Forearm 



Consists of two bones, one of which is called ulna, from its 

 being used as a measure ; and the other radius, from the sup- 

 posed resemblance to the spoke of a wheel. 



These bones are concerned in very different operations. The 

 ulna forms the elbow joint with the os humeri ; the radius is 

 the movable basis of the hand. 

 16* 



