THE ULNA. 281 



THE FOREARM. 



The skeleton of the forearm consists of two bones, — an inner, the ^dna, and an 

 outer, the radius. The former is large above and small below ; the latter, the con- 

 verse. The ulna plays around the trochlea in flexion and extension, carrying the 

 radius with it. The radius plays on the ulna in pronation and supination, carrying 

 with it the hand. These bones are connected by an interosseous membrane, which 

 gives origin to muscles, adds to the security of the framework, and yet implies a 

 great saving in weight. 



THE ULNA. 



The ulna consists of a shaft and two extremities. 



The upper extremity is devoted to the joint with the humerus, and laterally 

 to that with the head of the radius. The former articular surface is the greater sig- 

 moid cavity hollowed out of the continuous surfaces of the olecrano7i process behind 

 and above and the coronoid process in front. The olecranon/ a cubical piece of 

 bone projecting upward in continuation with tRe shaft, presents this articular surface 

 in front (to be described later), and a superior, a posterior, and two lateral surfaces. 

 The superior surface is pointed in front, with the point or beak external to the 

 middle. A slight groove just back of the edge serves for the attachment of the 

 capsular ligament. Behind this are two parts of different texture, the posterior of 

 which is for the insertion of the triceps. The posterior surface is triangular, bounded 

 above by the irregular edge of the top, and laterally by two lines which meet below 

 to make the posterior border of the shaft. It is subcutaneous, and is coveted by a 

 bursa (Fig. 294). The outer surface is bounded in front by the sharp edge of the 

 sigmoid cavity, along which is the groove for the capsule. Behind this is a hollow 

 for the anconeus. The i7i7ier surface has in front the inner border of the sigmoid, 

 less sharp than the outer, the capsular groove, and farther back a rough elevation. 

 The coronoid process ^ rises from the anterior surface of the front of the shaft. It 

 has an upper ^ articular surface, an anterior, and two lateral ones. The front surface 

 rises to a point nearer the outer side. The capsular groove runs along the border ; 

 and below this, bounded by two lines meeting below, is a rough region for the 

 brachialis anticus. Within the angle formed by the meeting of these two lines is a 

 rough rounded space, the tuberosity of the ulna, from the edge of which arises the 

 oblique ligament. The brachialis anticus is inserted into the lower part of this sur- 

 face and the tuberosity. The inner surf ace is bounded above by the sharp project- 

 ing border of the sigmoid cavity, at the edge of which is a rough prominence from 

 which certain fibres of the flexor sublimis digitorum take origin. The outer surface 

 presents the lesser sigmoid cavity. 



The greater sigmoid cavity' occupies the anterior surface of the olecranon and 

 the superior one of the coronoid process. There is a constriction in the middle of 

 both borders, but deeper in the outer, where the two processes meet, and the articu- 

 lar surface on the dry bone seems often to be interrupted in a line between them. 

 The sigmoid cavitv, concave from above downward, is broader in the upper half than 

 the lower. It is surrounded, except where it is joined by the lesser sigmoid cavity, 

 by an ill-marked groove for the capsular ligament. The articular surface is subdivided 

 by a rounded ridge, running from the point of the olecranon to that of the coronoid, 

 into a larger inner and a smaller outer portion. The course of this ridge is generally 

 somewhat inward as well as downward. This and the cross-line divide the articular 

 surface into four spaces. Of the upper, the inner is concave and the outer convex 

 from side to side. Of the lower, the inner is concave in the same direction and the 

 curve of the outer is uncertain ; probably, as a rule, slightly concave, it may be 

 plane or a little convex. 



The lesser sigmoid cavity,* for the head of the radius, is a concavity on the 

 outer side of the coronoid process, separated from the greater by a ridge, which 

 does not interrupt the cartilage coating both. It generally is an oblong quadrilateral 

 area forming about one-sixth of the circumference of a cylinder, with parallel borders ; 



^Olecranon. - Processus coronoideus. ^ Incisura semilanaris. ^ lacisura radialis. 



