174 THE VOYAGE OF H.M.S. CHALLENGER. 



ulnaris. It arises from the outer surface of the olecranon between the middle and posterior tubercles, 

 from the ulna posterior to the ridge running down the shaft from the middle tubercle, and from 

 the external surface of the ulna, where the ridge ends, to 1 inch from the epiphysial line. It has 

 also fibres of origin from the fascial expansion of the external lateral ligament, which passes 

 beneath it to attach itself to the lower end of the ridge of the ulna. It forms a flat strong tendon, 

 and crosses obliquely forwards and outwards to the interval between the radius and ulna. At the 

 level of the epiphysial line it enters the annular ligament, and passes over the posterior inferior 

 corner of the lower end of the radius. It reaches the carpus and goes between the 1st and 2nd 

 metacarpal bones ; at the middle of the 1st metacarpal it begins to expand to the outer or anterior 

 side, the tendon crossing the head of the metacarpal. Above the wrist it is crossed by the extensor 

 carpi ulnaris, and over the wrist by the common extensors. Upon the base of the 2nd meta- 

 carpal it crosses the posterior tendon of the extensor radialis, and then descends upon the dorsum 

 of the head of the 1st metacarpal, over which it joins the outer side of the extensor prirni 

 internodii pollicis. It is inserted into the base of the 1st phalanx of the pollex, and sends a fine 

 aponeurosis to the terminal one. The posterior capsule of the joint of the 1st and 2nd phalanges 

 of the pollex is chiefly formed by it. 



This muscle is wanting in the Phocinse, and comes under the heading " extensor pollicis et 

 indicis " in Dr. Murie's papers on the Pinnipedia. In Otaria it arises almost from the same locality 

 of the ulna, and is inserted into precisely the same part of the pollex as in Arctoccphalus. The surface 

 of the ulna which gives origin to this muscle in Arctocephalus, Otaria, and Trichcchus, is occupied 

 by the extensor primi internodii pollicis in the Phocinaa. It is supplied by the posterior inter- 

 osseous nerve. 



The Extensor ossis metacarpi pollicis is the abductor pollicis of Lucae. It arises from the 

 external flat surface of the olecranon, beginning above the tip of the olecranon, and extending along 

 its border to where the posterior border of the ulna begins; from the anterior upper half of the 

 outer surface of the ulna; slightly from the external lateral ligament; from the outer surface of the 

 interosseous ligament ; and by a few fibres from the middle of the posterior border of the radius. 

 After receiving the radial fibres it forms a flat tendon which descends obliquely downwards and 

 forwards over the outer surface of the radius, crossing the extensor carpi radialis tendon above the 

 wrist. It enters the first division of the annular ligament and runs in the groove on the anterior 

 border of the radius above the styloid process, then over the palmar surface of the process or 

 tubercle of the scapholunar bone; and is inserted into the radial or anterior side of the proximal 

 end of the 1st metacarpal bone. 



In ArctoccpJialus it arises from the external surface of the middle of the shaft of the ulna, 

 anterior to the ridge which descends from the middle tubercle, below the origin of the extensor 

 primi internodii pollicis, which is not covered by the attachment of the fascial expansion of the 

 external lateral ligament, from the posterior surface of the interosseous membrane, and by a small 

 surface from the external side of the posterior border of the radius in front of the ulnar origin. 

 It passes obliquely downwards and forwards, over the external surface of the radius, under cover 

 of the expansion of the external lateral ligament. Crossing the tendons of the extensor carpi radialis, 

 it enters the first division of the annular ligament on the anterior border and external surface of 

 the lower end of the radius, and runs along the side of the carpus to the outer side of the extensor 

 primi internodii pollicis; and is inserted into the radial side of the base of the 1st metacarpal. 



