AND PHYSIOLOGY. 



59 



as if they constituted a pivot ; Fig. * 5 - 



an instance of which is seen in 

 the turning of a screw, or in 

 the unlocking of a door. Of 

 these two bones the Ulna arti- 

 culates with the humerus, form- 

 ing only a ligamentous union 

 with the bones of the wrist. 

 It is prismoid in form, and is 

 of a hooked shape at its up- 

 per extremity, so that it 

 makes the union between it- 

 self and the humerus a very 

 secure one. The word ulna 

 is a Latin term signifying an 

 ell, because the forearm in 

 early times was used for that 

 measure. 



133. The Radius The 



Radius is the mate of the 

 ulna. Its upper extremity is 

 the smallest, and the lower 

 the largest, since its only true 

 articulation is at the wrist. 

 A firm membrane, however, 

 unites this bone to its fellow nearly its whole length. It prob- 

 ably derives its name from the fact that it measures the ra- 

 dius of a circle which may be described by the hand about 

 the elbow as a center. 



134. The Carpus. — The bones of the Carpus or Wrist are 

 eight in number, are small and irregular, and have the gen- 

 eral disposition of two rows. The first row, commencing with 

 the one nearest the thumb, contains the Scaphoid, Semilunar, 



In what operations do we need the two bones of the forearm ? Give the derivation of 

 ulna. 133. What is the mate of the ulna? Where does this articulate? What is the 

 derivation of its name ? 134. How many bones in the wrist? Give their names. 



Bones of the Forearm. 1, The Ulna. 

 2 and 3, The Sigmoid Notches. 4, The 

 Olecranon Process. 5, Coronoid Process. 

 6, Nutritious Foramen. 7. Ridge for at- 

 tachment of Interosseous Membrane. 8, 

 Capital um Ulnae. 9, Styloid Process. 10, 

 Shaft of the Radius. 11, 12 and 13, Head, 

 Neck, and Tuberosity of Radius. 14, Ob- 

 lique Line for muscular attachments. 15, 

 Styloid Process. 



