192 SPECIAL ANATOMY OF THE SKELETON 



The Lower Extremity is large, of quadrilateral form, and provided with two 

 articular surfaces one at the extremity, for articulation with the carpus, and one 

 at the inner side of the bone, for articulation with the ulna. The carpal articular 

 surface (fades articularis carpea) is of triangular form, concave, smooth, and divided 

 by a slight antero-posterior ridge into two parts. Of these, the external is of a tri- 

 angular form, and articulates with the scaphoid bone; the inner is quadrilateral 

 and articulates with the semilunar bone. The articular surface for the head of the 

 ulna is called the sigmoid cavity of the radius (incisura ulnaris); it is narrow, con- 

 cave, smooth, and articulates with the head of the ulna. The circumference of this 

 end of the bone presents three surfaces an anterior, external, and posterior. The 

 anterior surface, rough and irregular, affords attachment to the anterior ligament of 

 the wrist-joint. The external surface is prolonged obliquely downward into a strong 

 conical projection, the styloid process (processus styloideus), which gives attachment 

 by its base to the tendon of the Brachioradialis, and by its apex to the external 

 lateral ligament of the wrist-joint. The outer surface of this process is marked 

 by a flat groove, which runs obliquely downward and forward, and gives passage 

 to the tendons of the Extensor ossis metacarpi pollicis and the Extensor brevis 

 pollicis. The posterior surface is convex, affords attachment to the posterior 

 ligament of the wrist, and is marked by three grooves. Proceeding from without 

 inward, the first groove is broad but shallow, and subdivided into two by a slightly 

 elevated ridge; the outer of these two transmits the tendon of the Extensor carpi 

 radialis longior, the inner the tendon of the Extensor carpi radialis brevior. The 

 second, which is near the centre of the bone, is a deep but narrow groove, bounded 

 on its outer side by a sharply defined ridge; it is directed obliquely from above, 

 downward and outward, and transmits the tendon of the Extensor longus pollicis. 

 The third, lying most internally, is a broad groove, for the passage of the tendons 

 of the Extensor indicis and Extensor communis digitorum. 



Structure. Similar to that of the other long bones. 



Development (Fig. 149). From three centre?, one for the shaft and one for each extremity. 

 That for the shaft makes its appearance near the centre of the bone, about the eighth week of 

 fetal life. About the end of the second year ossification commences in the lower epiphysis, and 

 about the fifth year in the upper end. At the age of seventeen or eighteen the upper epiphysis 

 becomes joined to the shaft, the lower epiphysis becoming united about the twentieth year. Some- 

 times an additional centre appears about the fourteenth year in the bicipital tuberosity. 



Articulation. With four bones the humerus, ulna, scaphoid, and semilunar. 



Attachment of Muscles. To nine: To the tuberosity, the Biceps; to the oblique ridge, the 

 Supinator [brevis], Flexor sublimis digitorum, and Flexor longus pollicis; to the shaft (its anterior 

 surface), the Flexor longus pollicis and Pronator quadratus; (its posterior surface), the Extensor 

 ossis metacarpi pollicis and Extensor brevis pollicis; (its outer surface), the Pronator teres; and 

 to the styloid process, the Brachioradialis. 



Surface Form. Just below and a little in front of the posterior surface of the external con- 

 dyle a part of the head of the radius may be felt, covered by the orbicular and external lateral 

 ligaments. There is in this situation a little dimple in the skin, which i.s most visible when the 

 arm is extended, and which marks the position of the head of the bone. If the finger is placed 

 on this dimple and the forearm pronated and supinated, the head of the bone will be distinctly 

 perceived rotating in the lesser sigmoid cavity. The upper half of the shaft of the radius can- 

 not be felt, as it is surrounded by the fleshy bellies of the muscles arising from the external con- 

 dyle. The lower half of the shaft can be readily examined, though covered by tendons and 

 muscles and not strictly subcutaneous. If traced downward, the shaft will be felt to terminate 

 in a lozenge-shaped, convex surface on the outer side of the base of the styloid process. This 

 is the only subcutaneous part of the bone, and from its lower extremity the apex of the styloid 

 process will be felt bending inward toward the wrist. About the middle of the posterior aspect 

 of the lower extremity of the bone is a well-marked ridge, best perceived when the hand is slightly 

 flexed on the wrist. It forms the outer boundary of the oblique groove on the posterior surface 

 of the bone, through which the tendon of the Extensor longus pollicis runs, and serves to keep 

 that tendon in place. 



Applied Anatomy of the Radius and Ulna. The two bones of the forearm are more often 

 broken together than is either the radius or ulna separately. It is therefore convenient to consider 

 fractures of both bones in the first instance, and subsequently to mention the principal fractures 



