POST. BORDE 



THE RADIUS. 97 



supinator ridge, giving origin to a part of the supinator brevis muscle. Between 

 these lines, and below the small sigmoid cavity, is a triangular hollow, the fore part 

 of which lodges the tuberosity of the radius with the insertion of the biceps tendon in 

 pronation of the hand, while in the hinder part fibres of the supinator brevis arise. 

 The anterior surface is concave in the upper two-thirds, where the flexor profundus 

 digitorum muscle takes origin, and in its lower third is marked by an oblique line 

 the pronator ridge, which joins the anterior border and limits the attachment of the 

 pronator quadratus. Above the middle is a foramen for the medullary artery, 

 directed upwards. The internal surface is smooth and convex, in the upper two- 

 thirds giving attachment to the flexor profundus muscle, in the lower third 

 subcutaneous. The posterior surface, more uneven, looks outwards and backwards ; 

 an indistinct oblique line, descending from the supinator ridge to the posterior 

 border at the junction of the upper and middle thirds of the shaft, limits a trian- 

 gular area, which extends over the outer side of the olecranon and gives attachment 

 to the anconeus muscle ; below this a longitudinal ridge divides the surface into 

 an inner portion, smooth, and covered by the 

 extensor carpi ulnaris, and an outer part, 

 more irregular, and impressed by the ex- 

 tensor muscles of the thumb and index 

 fingers. 



The inferior extremity presents a 

 rounded head, from the inner and back 

 part of which a short cylindrical eminence, 

 the styloid process, projects downwards, 

 giving ' attachment in front and below to Fig. 96. TRANSVERSE SECTION THROUGH THE 

 the internal lateral ligament of the wrist- MIDDLE OF THE BOXES OF THE FOREARM, 



joint, and externally to the triangular fibro- 

 cartilage. The head bears two articular 

 surfaces, an inferior, semilunar and flat- 

 tened, upon which the triangular fibro-cartilage of the wrist plays ; and a lateral, 

 narrow and convex, which is received into the sigmoid cavity of the radius. 

 The head and the styloid process arc separated posteriorly by a groove, which is 

 traversed by the tendon of the extensor carpi ulnaris ; and inferiorly by a depres- 

 sion, into which also the triangular fibro-cartilage is inserted. 



The ulna is placed in its whole length under the skin at the back of the forearm. 

 The subcutaneous tract comprises the triangular surface on the back of the 

 olecranon, the posterior border of the shaft, which lies ab the bottom of a longi- 

 tudinal groove between the flexor and extensor muscles, and, in the lower third, a 

 narrow strip of the internal surface leading down to the styloid process : the latter 

 projects in the supine position of the hand at the inner and posterior part of the 

 wrist ; but when the hand is pronated, the outer and fore part of the head of the 

 ulna becomes superficial and prominent between the tendons of the extensor carpi 

 ulnaris and extensor minimi digiti muscles. 



THE KADIUS.i 



The radius is the external of the two bones of the forearm, and extends from 

 the humerus to the carpus. It articulates with the humerus, the ulna, and the 

 scaphoid and lunar bones of the carpus. 



The superior extremity or head, is disc-shaped, with a smooth vertical 



1 In anatomical description the forearm is supposed to be placed in supination, with the thumb 

 directed outwards and the palm of the hand looking forwards. 



