218 



THE SKELETON. 



The posterior surface is rounded, convex and smooth, in the upper third of its 

 extent, and covered by the Supinator Brevis muscle. Its middle third is broad, 

 slightly concave, and gives attachment to the Extensor Ossis Metacarpi Pollicis 

 above, the Extensor Primi Internodii Pollicis below. Its lower third is broad, 

 convex, and covered by the tendons of the muscles, which subsequently run in 

 the grooves on the lower end of the boue. 



The external surface is rounded and convex throughout its entire extent. Its 

 upper third gives attachment to the Supinator Brevis muscle. About its centre 

 is seen a rough ridge, for the insertion of the Pronator Radii Teres muscle. Its 

 lower part is narrow, and covered by the tendons of the Extensor Ossis Meta- 

 carpi Pollicis and Extensor Primi Internodii Pollicis muscles. 



The Lower Extremity of the radius is large, of quadrilateral form, and pro- 

 vided 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 is of triangular form, concave, smooth, and divided 

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

 large, of a triangular form, and articulates with the scaphoid bone ; the inner, 

 smaller and quadrilateral, articulates with the semilunar. The articular surface 

 for the ulna is called the sigmoid cavity of the radius; it is narrow, concave, 

 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 down- 

 wards into a strong conical projection, the styloid process, which gives attach- 

 ment by its base to the tendon of the Supinator Longus, and by its apex to the 

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

 is marked by two grooves, which run obliquely downwards and forwards, and 



are separated from one another by an 



Fig. 160. Plan of the Development of the 

 Radius. By Three Centres. 



Appear* a 



ulna. 



elevated ridge. The anterior one gives 

 passage to the tendon of the Extensor 

 Ossis Metacarpi Pollicis, the posterior 

 one to the tendon of the Extensor 

 Primi Internodii Pollicis. The poste- 

 rior surface is convex, affords attach- 

 ment to the posterior ligament of the 

 wrist, and is marked by three grooves. 

 Proceeding from without inwards, the 

 first groove is broad, but shallow, and 

 subdivided into two by a slightly elevat- 

 ed ridge ; the outer of these two trans- 

 mits 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, directed obliquely from above, 

 downwards and outwards; it transmits 

 the tendon of the Extensor Secundi 

 Internodii Pollicis. The third, lying 

 most internally, is a broad groove, for 

 the passage of the tendons of the Ex- 

 tensor Communis Digitorum, Extensor 

 Indicis and Extensor Minimi Digiti; 

 the tendon of the last-named muscle 

 through the groove at the point of articulation of the radius with the 







Structure. Similar to that of the other long bones. 



