80 BONES OF THE UPPER LIMB. 



prominent. 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 coronoid 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. 



Varieties. It is not uncommon to find a small hooklike process, with its point 

 directed downwards, placed in front of the internal supracondyloid ridge, the 

 supracondyloid process. From its extremity, a fibrous band, giving origin to the 

 pronator radii teres muscle, passes to the internal supracondyloid eminence, and 

 through the arch thus formed passes the median nerve, accompanied frequently by 

 the brachial artery, or by a large branch rising from it. This process represents a 

 portion of the bone completing a foramen in carnivorous animals. Much has been 

 written on the subject. (Struthers, Edin. Med. Journ. 1848. Gruber, " Canal is 

 supracondyloideus humeri," Mem. de 1'Acad. Imp. de St. Petersburg, 1859, p. 57. 

 Hyrtl, "Topogr. Anat." v. ii. p. 283.) 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 is thickest at its lower extremity. 



It articulates with the humerus, the ulna, the scaphoid, and the semi- 

 lunar bones. 



The head, or superior articular extremity, is of the form of a disc, with a 

 smooth vertical 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 on the small sigmoid cavity of the 

 ulna and within the orbicular ligament. The head is supported on a con- 

 stricted portion of a cylindrical form, named the neck. 



The shaft or body 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 in terosseous 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 

 medullary vessels, directed upwards into the bone. The posterior surface 

 presents slight oblique impressions of the extensor muscles of the thumb. 



The lower extremity of the radius, broad and thick, and somewhat quadri- 

 lateral, presents inferiorly a large surface, which articulates with the carpus, 

 and internally a small one, which articulates with the ulna. The carpal 



* When the arm is at rest it hangs naturally with the thumb directed forwards and 

 inwards, as in partial pronation ; but in anatomical description the forearm is placed as 

 in supination, with the thumb directed outwards and the palm of the hand looking 

 forwards. 



