694 



RADIUS. 



inner portion, called the trochlea, is the larger, and articu- 

 lates with the ulna. The outer receives the head of the 

 radius. Ahove the trochlea, and in front of the humerus, 

 is the anterior or lesser cavity for receiving the coronoid 

 process of the ulna. Behind the humerus, and above the 

 trochlea, is the posterior or greater cavity for receiving the 

 olecranon process of the ulna. Between these two cavities 

 the hone is transparent, and sometimes wanting. 



Structure. The humerus consists of compact structure 

 in its body, and is cellular at its extremities. It contains 

 a large medullary canal. It is articulated to the scapula, 

 radius and ulna. Its development is from eight points, viz : 

 one for the body, one for the head, one for each tuberosity, 

 one for the trochlea, one for each condyle, and one for the 

 small head. Ossification is noticed to begin shortly after 

 that of the clavicle. At birth the extremities are found 

 cartilaginous,, while the bony shaft is nearly complete. 

 From the close of the first and during the second and third 

 years, ossific centres are observed in the head and tuberosi- 

 ties during the third and sixth years the trochlea and 

 small head of the humerus during the fifth to the seven- 

 teenth year ossification appears in the condyles, and the 

 bone is found complete about the twentieth year. 



SECTION III. 

 BONES OF THE FOREARM THE RADIUS, (Fig. 218.) 



The radius is situated upon the outer side of the ulna, 

 the palm of the hand looking upward, and is the shorter 

 bone of the two occupying the forearm. 



It consists of a body and two extremities. The body or 

 shaft of the radius presents three surfaces, and is rather 

 triangular in shape. The anterior surface is covered below 

 by the pronator quadratus, and is there broad ; a little 

 Above its centre is seen the orifice of an oblique vascular 

 canal. The posterior surface is convex above and gives 

 attachment to the supinator brevis, and is concave in the 

 middle for the extensors of the thumb. The external 

 surface is round, and in its centre is rough for the insertion 

 of the pronator teres. 



