OPPOXEXS POLL/CIS 341 



2. OPPONENS POLLICIS 



The opponens pollicis, or flexor ossis metacarpi pollicis, is named from its 

 action, as it lu'lj)s in opposing the thumb to the other lingers and at the same time 

 is a liexor of tlie tirst metacarpal lione. It is a short, thick, triangular sheet. 



Origin. — (1) The front of the ridge on the trapezium below the preceding; and 

 (2) the lower and outer i)art of the front of the anterior annular ligament. 



Insertion. — The whole of the outer border of the anterior surface of the shaft 

 of the tirst metacarpal bone. 



Structure. — Arising ])y a short tendinous or fleshy origin, the muscular fibres 

 diverge fanwise to their insertion. The lower border of the muscle is often so 

 blended with the outer head of the next muscle that the separation is somewhat 

 artificial. 



Nerve-supply. — From the same source as the preceding; the filaments enter- 

 ing the anterior surface of the muscle near to the upper part of its ulnar border. 



Action. — To flex the first metacarpal bone, which it draws forwards and 

 inwards. This movement, on account of the shape of the carpo-metacari^al joint, 

 is accompanied by a certain amount of rotation inwards, by which the palmar 

 asi)ect of the thumb is made to look backwards and inwards. 



Relations. — Superficially, the abductor pollicis and the thenar fascia; deeply, 

 and upon its ulnar border, the flexor brevis pollicis. It also lies upon the joint 

 between the metacarpal bone and the trapezium. 



3. FLEXOR BREVIS POLLICIS 



The flexor brevis pollicis — named from its action and short length in compari- 

 son with the long flexor — consists of two heads: each forming a flat triangular 

 sheet. 



Origin. — The outer head arises from (1) the outer two-thirds of the lower 

 border of the anterior annular ligament; (2) the lower part of the ridge of the trape- 

 zium. The inner head, from (1) the front of the os magnum; (2) the front of the 

 bases of the first, second, and third metacarpal bones; and (3) from the front of the 

 sheath of the flexor carpi radialis tendon. 



Insertion. — The outer and inner sides of the front of the base of the first 

 phalanx of the thumb. 



Structure. — Arising by short tendinous as well as l)y fleshy filn'es, the two 

 heads become tendinous a short distance above the first metacarpo-phalangeal joint, 

 and have imbedded in their suljstance sesamoid bones of a somewhat hemispherical 

 shape, and about one-sixth of an inch (nearly 1 cm. ) in diameter, which rest by 

 small articular facets covered with cartilage upon the i)almar aspect of the condyles 

 of the first metacarpal bone. Close to their insertion the tendinous fil)res are 

 blended with those of the abductor pollicis externally, and the adductor internally. 

 In the inner head, three distinct divisions may generally be observed. The Jirst or 

 outermost division is a small fusiform slip which passes beneath the tendon of the 

 flexor longus pollicis to join the outer head, and to be inserted into the radial sesa- 

 moid bone. The second divisionis fan-shaped, and its fibres, which form the greater 

 part of the inner head, converge to embrace the ulnar sesamoid bone. The third 

 division lies under cover of the second. It is a small flesliy slip, which arises from 

 the ulnar side of the l^ase of the first metacarpal l^one, and passes downwards to he 

 inserted with the adductor pollicis into the ulnar side of the base of tlie first ])ha- 

 lanx. Sonu^ authors descril)e this third division as the inner head of the flexor 

 brevis pollicis, and the first and second divisions as the adductor pollicis 

 obliquus. 



Nerve-supply. — The outer head issupjdied (through the sixth cervical nerve) 

 l)y the palmar Viranch of tlie median, which enters the middle of its anterior surface 

 near its ulnar border. The inner head is supplied (through the eighth cervical 

 nerve) by the deep liranch of the ulnar nerve which enters the upper part of its 

 deep surface close to its ulnar border. 



