THE RADIU8 191 



of cartilage which is thinnest at its centre. Around the circumference of the head 

 is a smooth, articular surface (circwnferentia articular is], broad internally where 

 it articulates with the lesser sigmoid cavity of the ulna; narrow in the rest of its 

 circumference, where it rotates within the orbicular ligament. It is coated with 

 hyaline cartilage in the recent state. The head is supported on a round, smooth, 

 and constricted portion of bone, called the neck (collum radii), which presents, 

 behind, a slight ridge, for the attachment of part of the Supinator [brevis]. 

 Beneath the neck, at the inner and front aspect of the bone, is a rough eminence, 

 the bicipital tuberosity (tuberositas radii). Its surface is divided into two parts 

 by a vertical line a posterior, rough portion, for the insertion of the tendon of 

 the Biceps muscle; and an anterior, smooth portion, on which a bursa is inter- 

 posed between the tendon and the bone. 



The Shaft (corpus radii) is prismoid in form, narrower above than below, and 

 slightly curved, so as to be convex outward. It presents three surfaces, separated 

 by three borders. 



The anterior or palmar border (margo volaris) extends from the lower part of 

 the tuberosity above to the anterior part of the base of the styloid process below. 

 It separates the anterior from the external surface. Its upper third is very promi- 

 nent; and from its oblique direction, downward and outward, has received the 

 name of the oblique line of the radius. It gives insertion externally to the Supinator 

 [brevis] internally, it limits the origin of the Flexor longus pollicis, and between 

 these, arising from it, is the radial origin of the Flexor sublimis digitorum. The 

 middle third of the anterior border is indistinct and rounded. Its lower fourth is 

 sharp, prominent, affords attachment to the Pronator quadratus and to the poste- 

 rior annular ligament of the wrist, and terminates in a small tubercle at the base 

 of the styloid process, into which is inserted the tendon of the Brachioradialis. 



The posterior or dorsal border (margo dorsalis) commences above at the back 

 part of the neck of the radius, and terminates below at the posterior part of the 

 base of the styloid process; it separates the posterior from the external surface. 

 It is indistinct above and below, but well marked in the middle third of the bone. 



The internal or interosseous border (crista interossea) commences above at the 

 back part of the tuberosity, where it is rounded and indistinct, becomes sharp 

 and prominent as it descends, and at its lower part divides into two ridges, which 

 descend to the anterior and posterior margins of the sigmoid cavity. This 

 border separates the anterior from the posterior surface, and has the interosseous 

 membrane attached to it throughout the greater part of its extent. 



The anterior or palmar or flexor surface (fades volaris) is concave for its upper 

 three-fourths, and gives origin to the Flexor longus pollicis muscle; it is broad 

 and flat for its lower fourth, and gives attachment to the Pronator quadratus. 

 A prominent ridge limits the attachment of the Pronator quadratus below, and 

 between this and the inferior border is a triangular rough surface for the attach- 

 ment of the anterior ligament of the wrist-joint. At the junction of the upper 

 and middle third of this surface is the nutrient foramen, the opening of the 

 nutrient canal, which is directed obliquely upward (proximally). 



The posterior or dorsal or extensor surface (fades dorsalis) 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 origin to the 

 Extensor ossis metacarpi pollicis above, the Extensor brevis 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 bone. 



The external surface (fades lateralis) 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 teres 

 muscle. Its lower part is narrow, and covered by the tendons of the Extensor 

 ossis metacarpi pollicis and Extensor brevis pollicis muscles. 



