THE RADIUS. 



99 



\ T EBCL 



EXT. CARP. RAD. BRCV. 



T. CARP. RAD. LONG* 



articulates with the ulna. The carpal articular surface, concave and oblique, is 

 divided by a line into a quadrilateral internal part which articulates with the lunar 

 bone, and a triangular external part which articulates with the scaphoid bone. The 

 ulnar articular surface is placed at a right angle with the inferior surface, and is 

 concave from before backwards, forming the sigmoid cavity, which plays over the 

 rounded head of the ulna. To the smooth border between the radial and ulnar 

 articular surfaces the base of the triangular fibro -cartilage is attached. At the outer 

 part of the inferior extremity the styloid pi'ocess projects downwards, stout and 

 pyramidal, giving attachment to the external lateral ligament of the wrist-joint. 

 The posterior border of the lower articular surface descends farther than the anterior, 

 and is roughened for ligamentous attachment. Anteriorly, a prominent transverse 

 ridge forms the lower limit of the impression of the pronator quadratus, and between 

 this and the scaphoid articular facet is a small triangular area occupied by a strong 

 part of the anterior ligament of the wrist-joint. On its external and posterior 

 aspects the inferior extremity of the radim is marked by grooves, which transmit 

 the extensor tendons. Thus, on the external border is a flat groove directed down- 

 wards and forwards, which lodges the 

 extensor ossis metacarpi and extensor 

 brevis pollicis ; and on the posterior 

 surface are three grooves, the middle 

 one of which, oblique and narrow, and 

 bounded externally by a prominent 

 tubercle, lodges the extensor longus 

 pollicis ; while of the two others, 

 which are broad and shallow, the 

 external, subdivided by a slight ele- 

 vation, gives passage to the extensor 



j. ,. 1 . , . 



carpi radiahs longior and brevior, 



and the internal transmits the ex- 



tensor COmmunis digitorum and 



extensor indicis. Immediately above 



the first mentioned groove on the outer side is a small triangular mark, into which 



the tendon of the supinator longus is inserted. . 



The outermost groove is separated anteriorly from the impression of the pronator quadratus 

 by a prominent edge which is continued upwards from the transverse ridge of the front of 

 the lower extremity, and to which the posterior annular ligament is attached, while posteriorly 

 it is limited by a less marked elevation descending on the back of the styloid process. The 

 groove of the extensor longus pollicis is bounded externally by the tubercle, internally by a 

 small, often- indistinct, ridge ; and a sharp border projects between the innermost groove and 

 the sigmoid cavity. The several prominences give attachment to fibrous septa, which with 

 the annular ligament convert the grooves into canals for the passage of the tendons. 



The radius is for the most part deeply placed. The head and shaft are entirely 

 covered by muscles. At the lower end the styloid process comes to the surface 

 between the tendons of the extensor muscles of the thumb and forms a projection on 

 the outer side of the wrist, lower down than the styloid process of the ulna : the 

 tubercle on the back of the lower end of the bone may also be readily felt beneath 

 the skin. 



98. LOWEK EXTREMITIES OF RIGHT RADIUS AND 



ULNA , IN SU PI NATI ON, PROM BELOW. (Drawn by 

 T. W. P. Lawrence.) | 



The dorsal surface is uppermost. 



IracMal i 



The relative length of the forearm to the arm is expressed by the Uumero-radial or anti- 



ng : of _ adlus L x J l 2 \ the range of variation of which in man and the 

 - length of humerus / 

 anthropoid apes is shown by the following examples, viz. Eskimo, 71 ; European, 74 ; 

 Australian. 77 ; Negro, 79 ; Andamanese, 81 ; gorilla, 80 ; chimpanzee, 90 : and orang, 100. 

 The index is higher in the fcetus and infant, and diminishes during the period of growth. 



H 2 



