190 



SPECIAL ANATOMY OF THE SKELETON 



Olecranon. 



Appears <*~gjj^. Joins shaft at 

 10th year. j& ^ 16th year. 



Structure. -Similar to that of the other long bones. 



Development. From three centres one for the shaft, one for the inferior extremity, and 

 one for the olecranon (Fig. 148). Ossification commences near the middle of the shaft about 

 the eighth week, and soon extends through the greater part of the bone. At birth the ends are 

 cartilaginous. About the fourth year a separate osseous nucleus appears in the middle of the 

 head, which soon extends into the styloid process. About the tenth year ossific material appears 

 in the olecranon near its extremity, the chief part of this process being formed from an extension 

 of the shaft of the bone into it. At about the sixteenth year the upper epiphysis becomes joined 

 to the shaft, and at about the twentieth year the lower one. 

 Articulations. With the humerus and radius. 



Attachment of Muscles. To sixteen: To the olecranon, the Triceps, Anconeus, and one 

 head of the Flexor carpi ulnaris. To the coronoid process, the Brachialis anticus, Pronator teres, 



Flexor sublimis digitorum, and Flexor profundus digitorum; 

 generally also the Flexor longus pollicis. To the shaft, the 

 Flexor profundus digitorum, Pronator quadratus, Flexor carpi 

 ulnaris, Extensor carpi ulnaris, Anconeus, Supinator [brevis], 

 Extensor ossis metacarpi pollicis, Extensor longus pollicis, and 

 Extensor indicis. 



Surface Form. The most prominent part of the ulna on 

 the surface of the body is the olecranon process, which can 

 always be felt at the back of the elbow-joint. When the fore- 

 arm is flexed, the upper quadrilateral surface can be felt, 

 directed backward ; during extension it recedes into the olec- 

 ranon fossa, and the contracting fibres of the triceps prevent 

 its being perceived. At the back of the olecranon is the 

 smooth, triangular, subcutaneous surface, which below is 

 continuous with the posterior border of the shaft of the bone, 

 and felt in every position of the forearm. During extension 

 the upper border of the olecranon is slightly above the level 

 of the internal condyle, and the process itself is nearer to this 

 condyle than the outer one. Running down the back of the 

 forearm, from the apex of the triangular surface which forms 

 the posterior surface of the olecranon, is a prominent ridge of 

 bone, the posterior border of the ulna. This may be felt 

 throughout the entire length of the shaft of the bone, from the 

 olecranon above to the styloid process below. As it passes 

 down the forearm it pursues a sinuous course and inclines to 

 the inner side, so that, though it is situated- in the middle of 

 the back of the limb above, it is on the inner side of the wrist 

 at its termination. It becomes rounded off in its lower third, 

 and may be traced below to the small, subcutaneous surface of 

 the styloid process. Internal to this border the lower fourth 

 of the inner surface may be felt. The styloid process may be 



felt as a prominent tubercle of bone, continuous above with the posterior subcutaneous border 

 of the ulna, and terminating below in a blunt apex, which lies a little internal and behind, but 

 on a level with, the wrist-joint. The styloid process is best felt when the hand is in the same 

 line as the bones of the forearm, and in a position midway between supination and pronation. 

 If the forearm is pronated while the finger is placed on the process, it will be felt to recede, and 

 another prominence of bone will appear just behind and above it. This is the head of the ulna, 

 which articulates with the lower end of the radius and the triangular intra-articular fibrocarti- 

 lage, and now projects between the tendons of the Extensor carpi ulnaris and the Extensor 

 minimi digiti muscles. 



The Radius. 



The radius is situated on the outer side of the forearm, lying side by side with the 

 ulna, which exceeds it in length and size (Fig. 145). Its upper end is small, and 

 forms only a small part of the elbow-joint; but its lower end is large, and forms 

 the chief part of the wrist. It is one of the long bones, prismatic in form, slightly 

 curved longitudinally, and, like other long bones, has a shaft and two extremities. 



The Proximal or Upper Extremity presents a head, neck, and tuberosity. 



The head (capitulum radii) is of a cylindrical form, depressed on its upper 

 surface into a shallow cup (fovea capituli radii\ which articulates with the capitel- 

 lum or radial head of the humerus. In the recent state it is covered with a laver 



Joins shaft at 



Appears at 

 4th year. 



Inferior extremity. 



FIG. 148. Plan of the development 

 of the ulna. From three centres. 



