THE RADIUS. 259 



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 triangular 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; this is to 

 be felt in every position of the joint. 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 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 internal 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 

 external and above it. This is the head of the ulna, which articulates with the lower end of the 

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

 the Extensor carpi ulnaris and the Extensor minimi digiti 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. 

 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, where 

 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 

 by three borders. 



The anterior border extends from the lower part of the tuberosity above to the 

 anterior part of the base of the styloid process below. It separates the anterior 

 from the external surface. Its upper third is very prominent; and from its 

 oblique direction, downward and outward, has received the name of the oblique 

 lt'//>' of the radius. It gives attachment externally to the Supinator brevis, 

 internally to the Flexor longus pollicis, and between these to the Flexor sublimis 

 digitorum. The middle third of the anterior border is indistinct and rounded. Its 

 lower fourth is sharp, prominent, affords attachment to the Pronator quadratus, and 

 terminates in a small tubercle, into which is inserted the tendon of the Supinator 

 longus. 



The posterior border commences above at the back part of the neck of the 



