THE RADIUS. 155 



process being formed from an extension of the shaft of the bone into it. At 

 about the sixteenth year the upper epiphysis becomes joined, 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 radii 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 olecranon 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 conclyle 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 is to 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 sur- 

 face of the styloid process. Internal to this border the lower fourth of the inner surface is to 

 be felt. The styloid process is to 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 lowerend of the 

 radius and the triangular interarticular fibro-cartilage, and now projects between the tendons of 

 the Extensor carpi ulnaris and the Extensor minimi digit! muscles. 



The Radius. 



The Radius (radius, a ray, or spoke of a wheel) is situated on the outer side of 

 the forearm, lying side by side with the ulna, which exceeds it in length and size. 



* / O / O 



(Figs. 100 and 101.) 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 Upper Extremity presents a head, neck, and tuberosity. The head is of a 

 cylindrical form, depressed on its upper surface into a shallow cup which articulates 

 with the capitellum or radial head of the humerus. In the recent state it is covered 

 with a layer of cartilage which is thinnest at its centre. Around the circumference 

 of the head is a smooth, articular surface, broad internally where it articulates with 

 the lesser sigmoid cavity of the ulna ; narrow in the rest of its circumference, Avhere 

 it rotates within the orbicular ligament. It is coated with cartilage in the recent 

 state. The head is supported on a round, smooth, and constricted portion of bone, 

 called the neck, which presents, behind, a slight ridge, for the attachment of part 

 of the Supinator brevis. Beneath the neck, at the inner and front aspect of the 

 bone, is a rough eminence, the bicipital tuberosity. Its surface is divided into two 

 parts by a vertical line a posterior, rough portion, for the insertion of the tendon 

 of the Biceps muscle ; and an anterior, smooth portion, on which a bursa is 

 interposed between the tendon and the bone. 



The Shaft of the bone is prismoid in form, narrower above than below, and 

 slightly curved, so as to be convex outward. It presents three surfaces, separated 

 bv three borders. 



