380 PEOF. M. WATSON AND DE. A. H. YOUNG 



Posterior Humeral JRegion. 



Triceps Extensor Brachii. — This muscle possesses four distinct heads, 

 which arise as follows : — The outer head has an almost horizontal origin 

 extending from the lower border of the articular surface of the head 

 of the humerus, as far forwards as the outer surface of the base of the 

 great tuberosity. This head receives the fibres of insertion of the teres 

 minor. The middle head is the largest, and arises from the posterior 

 margin of the scapula for about two thirds of its length. A third head is 

 attached to the posterior surface of the shaft of the humerus immediately 

 above the olecranon fossa, whilst the fourth head has an oblique linear 

 origin from the inner side of the upper half of the shaft of the humerus. 

 The fibres derived from these different sources terminate on a single stout 

 tendon which is inserted into the olecranon process of the ulna, after giving 

 an offshoot to the fascia on the back of the forearm. The third head de- 

 scribed above corresponds to the anconeus {Epicondylo-olecranius). 



Muscles of the Forearm : Anterior Sadio-ulnar Region : Extensors. 



Anterior Extensor of Metacarpus is a broad fleshy muscle which arises 

 from the whole length of the external condyloid ridge of the humerus, and 

 also from the anterior part of the capsular ligament of the elbow-joint. It 

 is inserted by a stout tendon into the anterior border of the head of the 

 great metacarpal bone. 



Oblique Extensor of Metacarpus arises from the middle third of the ex- 

 ternal surface of the radius. The muscle crosses from without inwards, 

 and ends on a tendon which, passing underneath the extensor of the pha- 

 langes and superficial to the anterior extensor of the metacarpus, is inserted 

 into the inner small metacarpal bone. 



Anterior Extensor of Phalanges consists of two distinct portions. The 

 inner arises along with that portion of the anterior extensor of the meta- 

 carpus which is attached to the capsular ligament of the elbow-joint, and 

 forms a fusiform belly which terminates on a tendon opposite the lower end 

 of the radius. This tendon forms an expansion in front of the metacarpo- 

 phalangeal articulation, from which three bands pass to be inserted into 

 the inner, outer, and dorsal surfaces of the second phalanx of the inner of 

 the two anterior toes. The outer portion of the muscle arises from the 

 outer side of the external condyle of the humerus, by fleshy fibres from the 

 posterior aspect of the upper extremity of the radius, and from an aponeu- 

 rosis attached to the lower half of the posterior border of the ulna. From 

 this head a tendon is given off, which is distributed to the outer of the two 

 anterior digits, its insertion resembling that of the tendon of the same 

 muscle to the inner toe ; but in addition it also gives off, about the middle 

 of the metacarpus, a lateral slip which passes to the terminal phalanx of 

 the outer of the two posterior toes. 



Long Extensor of the Phalanges. — This arises by two heads, one of 



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