216 MUSCLES OF THE UrrER LIMB. 



commonest is the division of the tendon or lower part of the muscle into two 

 or sometimes three slips, which are inserted either togetlier or at some distance 

 from each other. The supinator longus may also be connected to some of the 

 neighbouring muscles, especially at its origin. 



The extensor carpi radialis longior muscle arises from the lower 

 part of the external coudylar ridge of the humerus, and from the inter- 

 muscular septum. Its muscular belly ends at the meeting of the upper 

 and middle thirds of the forearm in a flat tendon, which passes, con- 

 jointly with that of the following muscle, over the lower end of the 

 radius in the outermost of the two broad grooves on the posterior 

 surface of that bone, and is inserted into the base of the second 

 metacarpal bone. A small Inirsa lies beneath the tendon at its insertion. 



The extensor carpi radialis brevior muscle arises from the outer 

 condylar eminence of the humerus by a tendon common to it and 

 the other extensor muscles, from the intervening fibrous septa, from a 

 superficial tendinous expansion, and from the external lateral ligament 

 of the elbow-joint. Its muscular belly ends in a flat tendon, which 

 descends with that of the extensor longior, passes through the same 

 groove of the radius with it, and is inserted into the base of the meta- 

 carpal bone of the middle finger. A bursa is placed between this 

 muscle and the supinator brevis, and a smaller one between the tendon 

 and the metacarpal bone. 



Belatlons. — Of the two foregoing muscles the extensor longiTS is the most 

 superficial. The extensor brevis covers the supinator brevis and the insertion of 

 the pronator radii teres. The tendons of these muscles and that of the stipinator 

 longus are crossed obliquely by the extensors of the metacarpal bone and iirst 

 phalanx of the thumlj. 



Vavlct'u'g. — The two foregoing muscles are subject to a similar variation in 

 being, one or other of them, split \v^ into two or sometimes three tendons jire- 

 vious to insertion ; the tendons of either muscle being inserted into both the 

 second and third metacarpal bones : occasionally a slip passes also to the fourth 

 metacarpal bone. 



An additional muscle, called cxtcnftor carpi rndialis acc('.'<.W)'ii<s by Mr. Wood, is 

 of frequent occurrence, rising from the radius below the extensor carpi radialis 

 longior, and running through the same groove behind the radius at the wrist, it 

 is inserted most frequently into the metacarpal bone of the thrimb. but sometimes 

 into the abductor pollicis, first dorsal interosseous muscle or other part. 



The extensor communis digitoruni muscle, placed between the 

 extensor carpi radialis brevier and the extensor minimi digiti, arises from 

 the outer condylar eminence by a tendon common to it and the other 

 superficial extensor muscles, also from the fascia of the forearm, and the 

 septa between it and the adjoining muscles. Somewhat below the middle 

 of the forearm the muscular part ends in four tendons, which pass between 

 the posterior annular ligament of the wrist and the innermost broad 

 groove on the back of the radius, and diverge as they proceed along 

 the carpus and metacarpus to reach the fingers. Here each is increased 

 b}'' tendinous fibres derived from the lumbricalcs and interosseous 

 muscles forming a fibrous expansion, which covers the back of the first 

 and second digital phalanges, and terminates upon the third phalanx. 

 It is attached to tlie second and third phalanges in the following 

 manner. Opposite the first bone the tendon divides into three fasci- 

 culi ; the central one is much thinner than the others, and is inserted 

 into the base of tiie second phalanx ; the two lateral parts, continuing 



