THE MUSCULAR SYSTEM. 155 



EXTENSOR Ossis METACARPI POLLICIS. Origin, middle 

 third of posterior surface of radius, posterior surface of lower 

 three-fourths of shaft of ulna, and interosseous ligament; inser- 

 tion, into base of metacarpal of thumb; action, extends meta- 

 carpal of thumb ; nerve, posterior interosseous. 



EXTENSOR PRIMI INTERNODII (LONGUS) POLLICIS. Origin, 

 from the interosseous membrane and from the posterior aspect 

 of the shaft of the radius; insertion, into the base of the first 

 phalanx of the thumb; action, extends the thumb; nerve, pos- 

 terior interosseous. 



EXTENSOR SECUNDI INTERNODII (BREVIS) POLLICIS. Ori- 

 gin, from the posterior aspect of the shaft of the ulna and from 

 the interosseous membrane; insertion, into the last phalanx of 

 the base of the thumb; action, extends the thumb; nerve, pos- 

 terior interosseous. 



EXTENSOR INDICIS. Origin, from the posterior aspect of 

 the ulna and from the interosseous membrane; insertion, into 

 the second and third phalanges of the index finger, along with 

 the tendon of the extensor communis ; action, extends the index 

 finger; nerve, posterior interosseous. 



MUSCLES OF THE HAND. 



These are divided into three groups the radial region, the 

 ulnar region, and the palmar region. 



Radial Region. 



ABDUCTOR POLLICIS. Origin, from the annular ligament 

 and ridge of the trapezium; insertion, into base of the first 

 phalanx of the thumb on its radial side; action, abducts the 

 thumb from the median line; nerve, median. 



OrpONENS POLLICIS. Origin, from annular ligament and 

 palmar surface of the trapezium ; insertion, into metacarpal bone 

 of the thumb throughout the whole length of its radial side; 

 action, flexes the first metacarpal bone; nerve, median. 



FLEXOR BREVIS POLLICIS. This muscle consists of two por- 

 tions, divided by tendon of flexor longus pollicis; origin, super- 

 ficial portion from outer two-thirds of annular ligament and 

 trapezium, deeper portion (by some called adductor obliquus 

 pollicis) from sheath of flexor carpi radialis, the trapezoid, os 

 magnum, and base of first, second, and third metacarpal bones ; 

 insertion, into either side of base of first phalanx of thumb, tlic 

 inner portion joining the adductor and the outer portion the 

 abductor; each tendon has a sesamoid bone developed in it; 

 action, adducts thumb toward % median line; nerves, ulnar and 

 median. 



