THE RADIUS. 289 



a smooth margin to turn in the socket formed by the lesser sigmoid cavity and the 

 orbicular ligament, which completes it. The term "circular" is not used with 

 mathematical precision, for slight variations from it are the rule. The most common 

 one is an increase of the antero-posterior diameter. The depression on top is not 

 symmetrical, for there is almost invariably a greater thickness of the rim in front, 

 extending more to the inner than to the outer side. The smooth margin has a 

 downward projection internally. The plane of the upper surface is not always at 

 right angles to the axis of the neck, but often looks a little outward. The neck is 

 a smooth constricted portion some two centimetres in length and approximately 

 cylindrical. 



The shaft ' immediately bends outward below the neck, and has a slight forward 

 curve at the lower end, where it broadens considerably. The bicipital tuberosity ^ is 

 a large prominence at the inner and front aspect of the shaft, just below the neck. 

 Its posterior border, which is rough and projecting, slants forward and receives the 

 biceps tendon. In front of this the tuberosity is smooth for a bursa, lying beneath 

 the tendon, which, in pronation, is rolled around it. The shaft is described as having 

 three surfaces separated by three borders ; there is convenience in retaining the 

 plan, although only one border is always distinct and one is almost imaginary. 

 The distinct border is the interyial or interosseous^^ which, starting from the bicipital 

 tuberosity, soon becomes sharp for the interosseous membrane, and runs to the 

 lower quarter of the bone, where it divides into two descending lines to the front 

 and back of the articular facet on the inner side of the lower end. The miterior 

 border^ which is generally distinct above, starts from the front of the tuberosity and 

 runs downward and outward to about the middle of the bone. This part is known 

 as the oblique line of the radius, which gi\-es origin to a part of the flexor suLlimis 

 digitorum, and separates the insertion of the supinator brevis from the origin of the 

 flexor longus pollicis. The border is thence poorly marked till, slanting forward tc» 

 the beginning of the lower fourth, it becomes a distinct ridge running to the front of 

 the styloid process and receives the insertion of the pronator quadratus. It broadens 

 at the end into a triangular tubercle for the insertion of the brachio-radialis. The 

 posterior border is important only as helping to define the posterior and outer sur- 

 faces ; it is usually to be seen in the middle third of the bone, and has neither a 

 definite beginning nor end. The anterior szirface, limited above by the oblique line, 

 is slightly concave, and gives origin to the flexor longus pollicis as far down as the 

 last quarter, which is slightly hollowed for the pronator quadratus and sometimes 

 separated from the upper part by an oblique ridge. The nutrient forameji is seen 

 above the middle, running upward. The outer surface, which is convex, presents 

 about the middle a roughness for the insertion of the pronator radii teres. The 

 posterior surface has a concavity in the middle third, internal to the posterior bordei, 

 and is convex both above and below. 



The lower extremity bends slightly forward, ending in front in a prominent 

 ridge to which the capsule is attached. The outer side is prolonged downward as 

 the styloid process, ending in a blunt point. It usually shows grooves for the tendons 

 of the extensors of the metacarpal bone and first phalanx of the thumb, which pass 

 over it. The external lateral ligament of the wrist arises from it. The posterior 

 surface has a groove at its edge for the capsule, and above this is furrowed for the 

 passage of certain tendons. Next to the styloid process is a broad depression, 

 sometimes faintly divided into two, for the extensores carpi radialis longior et 

 brevior ; internal to this is a marked ridge, the tubercle, slanting downward and 

 feutward, with a narrow, deep gutter beyond it for the tendon of the long extensor 

 of the thumb. A very slight border separates this internally from a broad, shallow 

 groove for the tendons of the extensor communis and that of the index-finger. At 

 the extreme limit of the posterior surface is sometimes a minute furrow for a part 

 of the tendon of the extensor of the little finger, which passes over the radio-ulnar 

 joint. The inner side of the lower end is occupied by a concave articular area, the 

 sigmoid cavity^ of the radius, which receives the head of the ulna and much resembles 

 the lesser sigmoid cavity of that bone. The lower surface is articular for the scaph- 

 oid and semilunar bones of the wrist. It is in the main triangular, the base being 

 the inner side. It is overhung both before and behind, and is continued onto the 



* Corpus. - Tuberositas radii. ^ Crista interossea. ■* Margo volaris. ^ Incisura ulnaris. 



19 



