THE RADIUS. 87 



of the bone. Anteriorly, the internal margin of the trochlea is the 

 most prominent, and widens below into a convexity parallel to the 

 groove ; posteriorly, the external margin is most j^rominent. Above 

 the trochlea posteriorly is a large and deep pit, the olecranon fossa, 

 which receives the olecranon process of the ulna in extension of the 

 forearm ; and above it anteriorly, separated from the olecranon fossa 

 only by a thin lamina of bone, is the much smaller cor onoid fossa, 

 which receives the coronoid process in flexion. Above the capitellum 

 is a shallow depression, into which the head of the radius is pressed 

 in complete flexion. 



Varitfif.'i. — A small hook-like process, with its point directed downwards , is 

 not unfrequently found in front of the internal condylar ridge, the siq»-acon- 

 di/loid jjrocfs.s: From its extremity, a fibrous band, giving origin to the pronator 

 radii teres mnscle. passes to the internal condylar eminence, and through the 

 arch thus formed passes the median nerve, accompanied fi-equently by the brachial 

 artery, or IJV,^ large branch rising from it. This process represents a portion of 

 the bone eomplPTiir!^ a foi-amen in carnivorous animals. (See Struthers, Edin. 

 Med. Joum., 1848 : Gruber, " Canalis supracondyloideus hmneri," Mem. de TAcad. 

 Imp. de St. Petersljurg, 18.59, p. 57.) The thin plate between the olecranon and 

 coronoid fossae is sometimes perforated. 



THE RADIUS.* 



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, the scaphoid, and the semilunar bones. 



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

 tical margin. It presents on its summit a depression, which articulates 

 with the capitellum of the humerus, and is surrounded by a convex 

 part, broadest internally where it glides upon the groove internal to 

 the capitellum. The smooth, short, cylindrical surface of the vertical 

 margin, likewise broadest internally, rolls in the small sigmoid cavity 

 of the ulna, and within the orbicular ligament. The head is supported 

 on a constricted portion, named the neck. 



The shaft or bodi/ is slightly curved, with the convexity directed 

 outwards and backwards. On its internal aspect superiorly, where it 

 is continuous with the neck, is the bicipital tuberosity, to the posterior 

 border of which is attached the tendon of the biceps muscle. Below 

 the bicipital tuberosity the shaft presents three sides, the external of 

 which is rounded into the others by smooth convex margins, while the 

 anterior and posterior surfaces are separated by an acute internal 

 margin, which gives attachment to the interosseous-ligament. The 

 external surface is convex transversely as well as longitudinally ; it is 

 marked near the middle by an oval rough part, about one inch and a 

 half long, which gives insertion to the pronator radii teres. The 

 anterior surface is marked in its upper part by an oblique ridge, below 

 which is a shallow longitudinal groove for the flexor longus pollicis ; 

 inferiorly it is expanded, and presents a flat impression corresponding 

 with the pronator quadratus ; and above the middle is the foramen for 

 the medttllary vessels, directed upwards into the bone. The 2'osterior 

 surface presents slight oblique impressions of the extensor muscles of 

 the thumb. 



The lower extrcmitij of the radius, broad and thick, and somewhat 



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

 the tbumb directed outwards and the palm of tlie hand looking forwards. 



