148 ANATOMY OF UPPER EXTREMITY, ETC. 



seous membrane at its upper part, and appears between the 

 supinator brevis and the extensor ossis metacarpi muscles ; 

 it descends between the deep and superficial layers of mus- 

 cles, supplying them with muscular branches, and anasto- 

 moses with the posterior carpal arteries of the radial and 

 ulnar, and with the terminal twigs of the anterior inter- 

 osseous ; at its upper part it gives off a recurrent branch, 

 which passes beneath the anconeus muscle to supply the 

 elbow-joint, and anastomose with a branch of the superior 

 profunda of the brachial. 



The posterior interosseous nerve is given off, in front of 

 the outer condyle, from the musculo-spiral nerve ; it passes 

 through the fibres of the supinator brevis to descend the 

 back of the arm between the two layers of muscles as far as 

 the middle of the forearm, where it sinks beneath the ex- 

 tensor secundi internodii pollicis, and is distributed to the 

 back of the carpus; sometimes it has a gangliform swelling 

 at its termination. 



The SUPINATOR BREVIS MUSCLE is a small muscle at the 

 upper part of the arm, the fibres of which pass obliquely 

 round the upper third of the radius ; it arises from the ex- 

 ternal lateral and the orbicular ligaments, and from about 

 two inches of the upper part of the ulna, and is inserted 

 into the oblique line on the upper part of the radius, except 

 at its inner part. 



The EXTENSOR Ossis METACARPI POLLICIS lies next below 

 the supinator brevis, and is sometimes united with it ; it 

 arises from the posterior surface of the radius, from the 

 ulna, and from the intervening interosseous membrane ; it 

 forms a large belly, the tendon of which, passing through 

 the outer compartment of the posterior annular ligament, 

 and in a groove of the radius common to it and the exten- 

 sor primi internodii pollicis, is inserted into the base of the 

 metacarpal bone of the thumb. 



The EXTENSOR PRIMI INTERNODII POLLICIS is the smallest 

 muscle of the deep la} T er, and its tendon is closely connected 

 with that of the preceding muscle ; it arises from the radius 

 and interosseous membrane just below ths origin of that 

 muscle, and passing through the same compartment in the 

 annular ligament, is inserted into the base of the first pha- 

 lanx of the thumb. 



The EXTENSOR SECUNDI INTERNODII POLLICIS lies next 

 below, and is partly covered by the preceding muscle ; it 

 arises from the posterior surface of the ulna, and from the in- 



