70 NATURAL THEOLOGY-. 



use of the limb, two motions are wanted : a motion at the 

 elbow, backward and forward, which is called a reciprocal 

 motion ; and a rotary motion, by which the palm of the 

 hand, as occasion requires, may be turned upward. How is 

 this managed ? The fore-arm, it is well known, consists ol 

 two bones, lying alongside each other; but touching only to- 

 wards the ends. One, and only one of these bones is joined 

 to the humerus, or upper part of the arm, at the elbow ; the 

 other alone to the hand, at the wrist. The first, by means. 

 at the elbow, of a hinge-joint — which allows only of motion 

 in the same plane — swings backward and forward, carrying 

 along with it the other bone and the whole fore-arm. In 

 the mean time, as often as there is occasion to turn the palm 

 upward, that other bone to which the hand is attached rolls 

 upon the first by the help' of a groove or hollow near each 

 end of one bone, to which is fitted a corresponding promi- 

 nence in the other. If both bones had been joined to the 

 humerus (upper arm) at the elbow, or both to the hand, at 

 the wrist, the thing could not have been done. The first 

 was to be at liberty at one end, and the second at the other, 

 by which means the tAvo actions may be performed togeth- 

 er.* The great bone, which carries the fore-arm, may be 

 swinging upon its hinge at the elbow at the very time that 

 the lesser bone, which carries the hand, may be turning 

 round it in the gi'ooves. The management, also, of these 

 grooves, or rather of the tubercles and grooves, is very ob- 

 servable. The two bones are called the radius, and the 

 ulna. Above, that is, towards the elbow, a tubercle of the 



scZ joint; c, the elbow, exemplifying the hinge-joint ; d, the radius, 

 and e, the ulna. The radius belong^s more peculiarly to the wrist, be- 

 ing the bone which supports the hand, and turns with it in all its 

 revohing motions. The ulna belongs chiefly to the elbow-joint, and 

 by it we perform all the actions of bending the arrn and extending the 

 tore-arm. 



* Plate IL, Fig. 3, shows the connection of the radius, d, with 

 tbe ulna, e, at the elbow; a, being the humerus. The mode of aniicn 

 lation at the wrist i.s seen in Fig. 2. 



