396 THE MUSCLES AND FASCIAE. 



by its inner border, above the elbow, with the Brachialis anticus, the musculo- 

 spiral nerve, and radial recurrent artery ; and in the forearm with the radial 

 vessels and nerve. 



The Extensor carpi radialis longior is placed partly beneath the preceding 

 muscle. It arises from the lower third of the external supracondylar ridge of 

 the humerus, and from the external intermuscular septum by a few fibres from 

 the common tendon of origin of the Extensor muscles of the forearm. The 

 fibres terminate at the upper third of the forearm in a flat tendon, which runs 

 alono- the outer border of the radius, beneath the extensor tendons of the thumb; 

 it then passes through a groove common to it and the Extensor carpi radialis 

 brevior, immediately behind the styloid process, and is inserted into the base of 

 the metacarpal bone of the index finger, on its radial side. 



Eelations. By its superficial surface, with the Supinator longus and fascia 

 of the forearm ; its outer side is crossed obliquely by the extensor tendons of th 

 thumb ; by its deep surface, with the elbow-joint, the Extensor carpi radialis 

 brevior, and back part of the wrist. 



The Extensor carpi radialis brevior is shorter, as its name implies, and thicker 

 than the preceding muscle, beneath which it is placed. It arises from the 

 external condyle of the humerus by a tendon common to it and the three 

 following muscles; from the external lateral ligament of the elbow-joint, from 

 a strong aponeurosis which covers its surface, and from the intermuscular 

 septa between it and the adjacent muscles. The fibres terminate about the 

 middle of the forearm in a flat tendon which is closely connected with that of 

 the preceding muscle, and accompanies it to the wrist, lying in the same groove 

 on the posterior surface of the radius ; it passes beneath the extensor tendons of 

 the thumb, then beneath the annular ligament, and, diverging somewhat from its 

 fellow, is inserted into the base of the metacarpal bone of the middle finger, on 

 its radial side. 



The tendons of the two preceding muscles pass through the same compartment 

 of the annular ligament, and are lubricated by a single synovial membrane, but are 

 separated from each other by a small vertical ridge of bone as they lie in the 

 groove at the back of the radius. 



Relations. By its superficial surface, with the Extensor carpi radialis longior, 

 and with the Extensor muscles of the thumb which cross it ; by its deep surface, 

 with the Supinator brevis, tendon of the Pronator radii teres, radius, and wrist- 

 joint; by its ulnar border, with the Extensor communis digitorum. 



10. Posterior Radio-ulnar Region (Fig. 235). 



The muscles in this region are divided for purposes of description into two 

 groups or layers, superficial and deep. 



Superficial Layer. 



Extensor communis digitorum. Extensor carpi ulnaris. 



Extensor minimi digiti. Anconeus. 



The Extensor communis digitorum is situated at the back part of the forearm. 

 It arises from the external condyle of the humerus by the common tendon, from 

 the deep fascia, and the intermuscular septa between ft and the adjacent muscles. 

 Just below the middle of the forearm it divides into three fleshy masses, from 

 which tendons proceed; these pass, together with the Extensor indicis, through 

 a separate compartment of the annular ligament, lubricated by a synovial mem- 

 brane. The tendons then diverge, the innermost one dividing into two ; and all, 

 after passing across the back of the hand, are inserted into the second and third 

 phalanges of the fingers in the following manner : the outermost tendon, accom- 

 panied by the Extensor indicis, goes to the index finger; the second tendon is 

 sometimes connected to the first by a thin transverse band, and receives a slip 

 Irom the third tendon ; it goes to the middle finger ; the third tendon gives off 



