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PRACTICAL ANATOMY. 



Observe that the collective origin of the three thumb extensors practically 

 occupies the middle posterior third of ulna, radius, and interosseous membrane. 

 Compare the origins of these muscles as you find them on the cadaver, with the 

 origins as indicated in figure 246. 



Now locate (i) the posterior interosseous artery. (Fig. 247.) It lies between 

 the superficial and deep groups of muscles. It is a branch of the common inter- 

 osseous branch of the ulnar. Find the space between the oblique ligament and 

 the interosseous membrane, where this vessel appears. Also find where it anasto- 

 moses with the anterior interosseous artery below. Notice that in its course this 

 artery crosses the deep group of muscles, and is attended in a part of its course 

 only by the posterior interosseous nerve. Trace branches from this artery to all 

 the muscles in this locality. This artery is attended by the posterior interosseous 

 nerve in the upper part of its course. Show the exact relation of artery and nerve 

 on your work. (2) Locate the radial artery in the pulse region of the wrist. You 



FIG. 248. THE DORSAL INTEROSSEI. 



will find it passing under the three extensor muscles of the thumb, where it 

 pierces the first interosseous muscle and disappears into the deep palm. 



The Dorsal Interosseous Muscles (Figs. 247 and 248). Study the muscles 

 and their blood-supply. Find four of these muscles. Notice that they arise by 

 two heads from the contiguous surfaces of the metacarpal bones. Trace their 

 tendons of insertion to the tendon of the extensor communis digitorum. These 

 muscles (Fig. 248), as you might infer from their insertion, acting alone, are 

 abductors of the fingers. Acting with the palmar interossei and lumbricales, 

 they produce flexion at the metacarpo-phalangeal and extension at the phalan- 

 geal articulations. Acting in this way, they place the fingers in position for hold- 

 ing the pen. These muscles are all supplied by the ulnar nerve. 



The Supinator Radii Brevis. Detach all the muscles that arise from the 

 outer condyle : The extensor carpi radialis brevior, the extensor communis 

 digitorum, the extensor minimi digiti, and the extensor carpi ulnaris. Do this 



