PRACTICAL CONSIDERATIONS : THE FOREARM. 603 



Relations. — The relations of the muscle are essentially the same as those of 

 the extensor ossis metacarpi poUicis. 



Variations. — The extensor brevis and the metacarpal extensor of the thumb are differen- 

 tiations of a common muscle and show indications of this in their partial or complete fusion. 

 The tendon of the extensor brevis is sometimes continued onward to the terminal phalanx of 

 the thumb or may send a slip to the base of the second metacarpal. 



4. Extensor Longus Pollicis (Fig. 580). 



Attachments. — The long extensor of the thumb (m. extensor pollicis longus), 

 also known as the extensor secundi internodii pollicis, is an elongated fusiform mus- 

 cle lying along the medial border of the extensor brevis pollicis, which it partly 

 covers. It arises from the interosseous membrane and posterior surface of the ulna ; 

 its tendon passes downward in the third compartment beneath the posterior annular 

 ligament and, crossing over the tendons of the extensores carpi radiales, is itiserted 

 into the base of the terminal phalanx of the thumb. 



Nerve-Supply. — By the posterior interosseous branch of the musculo-spiral 

 nerve from the sixth, seventh, and eighth cervical nerves. 



Action. — To extend the terminal phalanx of the thumb and, continuing its 

 action, to extend and at the same time slighdy adduct the thumb. 



5. Extensor Indicis (Fig. 580). 



Attachments. — The extensor of the index-finger (m. extensor indicis proprius; 

 lies along the medial border of the extensor longus pollicis. It arises from the in- 

 terosseous membrane and the dorsal surface of the ulna. Its tendon passes, along 

 with the tendons of the extensor communis digitorum, through the fourth compart- 

 ment beneath the posterior annular ligament, and eventually is inserted with the 

 tendon of the common extensor which passes to the index-finger. 



Nerve-Supply. — By the posterior interosseous branch of the musculo-spiral 

 nerve from the seventh and eighth cervical nerves. 



Action. — To extend the index-finger. 



Variations. — The extensor indicis may be wanting, or its tendon may send slips to the 

 third and fourth digits. Occasionally a muscle arises from the ulna, below the origin of the ex- 

 tensor indicis, and passes to the third or fourth finger, forming what has been termed the extensor 

 digiti medii {vel annularis) proprius. This muscle represents an additional portion of the deep 

 extensor layer which normally disappears. 



PRACTICAL CONSIDERATIONS : THE FOREARM. 



The fascia descending from the arm to the forearm should be studied anteriorly 

 with relation to the expansion known as the bicipital aponeurosis (Fig. 570), — one 

 of the " two inferior tendons of the biceps" of the older anatomists, — which becomes 

 continuous with the deep fascia of the forearm, and thus, through the origin from its 

 under surface of fibres of many of the superficial muscles of that region, associates 

 their action with that of the biceps itself. Partly for this reason injuries and diseases 

 affecting the bicipital region are sometimes associated wath a certain weakness of 

 grasp and feebleness of wrist flexion. The facts that only this aponeurotic expansion 

 separates the median basilic vein from the brachial artery, and that in persons of poor 

 muscular development it is often so thin as scarcely to constitute a recognizable 

 layer, were of practical importance when phlebotomy of the median basilic was fre- 

 quent. Arterio-venous aneurism from accidental puncture of the artery was then 

 quite common. 



Posteriorly the outer aponeurotic expansion of the triceps, running over the 

 anconeus to become continuous with the deep fascia of the forearm, is of importance 

 in its relation to the power of extension of the forearm after excision of the elbow 

 (page 308). _ 



The fascia of the forearm, besides giving origin to many fibres of the subjacent 

 muscles, as has been noted above, envelops the forearm completely, being continu- 



