370 MUSCLES ON THE BACK OF THE FOREARM. 



8. Extensor Ossis Metacarpi Pollicis Manus. 

 Arises, fleshy, from the middle and posterior part of the ulna, 

 immediately below the insertion of the anconeus muscle, from 

 Fig. 97.* the posterior part of the middle of the radius, 

 and from the interosseous ligament. 



Inserted, generally by two tendons, into 

 the os trapezium, and upper back part of the 

 metacarpal bone of the thumb, and often 

 joins with the adductor pollicis. 



Use. To extend the metacarpal bone of 

 the thumb, outwardly. 



9. Extensor Primi Internodii, (Ext. 



Major Pollicis Manus,) 



Arises, fleshy, from the posterior part of 

 the ulna near the former muscle, and from 

 the interosseous ligament. 



Inserted, tendinous, into the posterior part 

 of the first bone of the thumb ; and a part of 

 it may be traced as far as the second bone. 



Use. To extend the first bone of the 

 thumb obliquely outwards. 



10. Extensor Secundi Internodii, (Ext. 



Minor Pollicis Manus,) 



Arises, by an acute, tendinous, and fleshy 

 beginning, from the middle back part of the ulna, and from the 

 interosseous ligament ; its tendon runs through a small groove at 

 the inner and back part of the lower end of the radius. 



Inserted into the last bone of the thumb. 



Use. To extend the last joint of the thumb obliquely back- 

 wards. 



* The deep layer of muscles on the posterior aspect of the fore-arm. 1. The 

 lower part of the humerus. 2. The olecranon. 3. The ulna. 4. The anconeus 

 muscle. 5. The supinator brevis muscle. 6. The extensor ossis metacarpi 

 pollicis. 7. The extensor primi internodii pollicis. 8. The extensor secundi 

 internodii pollicis. 9. The extensor indicis. 10. The first dorsal interosseous 

 muscle. The other three dorsal interossei are seen between the metacarpal 

 bones of their respective fingers. 



