EXTENSORS OF DIGITS. 285 



Acting with the long extensor of the wrist it will move the lower end 

 of the radius in supination. 



The EXTENSOR COMMUNIS DiGiTORUM, D, is single at its origin, but is 

 divided inferiorly into four tendons. It arises from the common tendon, 

 from aponeurotic septa between it and the muscles around, and from the 

 aponeurosis of the limb. Near the lower part of the forearm the muscle 

 ends in three tendons, which pass through a compartment of the annular 

 ligament with the indicator muscle; below the ligament, the most internal 

 tendon divides into two, and all four are directed along the back of the 

 hand to their insertion into the two last phalanges of the fingers. 



On the back of the fingers the tendons have the following arrangement. 

 On the dorsum of the first phalanx each forms an expansion with the ten- 

 dons of the lumbricales and interossei muscles (fig. 82). At the front of 

 that phalanx it divides into three parts (fig. 87, N ) : the central one is fixed 

 into the base of the second phalanx, whilst the lateral pieces unite, and are 

 inserted into the base of the last phalanx. On the fore and little fingers 

 the expansion is joined by the special tendons of those digits ; and oppo- 

 site the first two articulations of each finger the tendon sends down lateral 

 bands to join the capsule of the joint. 



This muscle is placed between the extensors of the wrist and little finger, 

 and conceals the deep layer. On the back of the hand the tendons are 

 joined by cross pieces, L, which are strongest between the ring finger ten- 

 don and its collateral tendons ; they prevent the ring finger being raised if 

 the others are closed. 



Action. The muscle straightens the three phalanges of the fingers from 

 root to tip, and separates the four digits from each other. It can extend 

 the nearest joint of each finger whilst the two farthest are kept bent by the 

 flexors; and it can straighten the last two articulations when the nearest 

 is bent. 



The digits being straightened, it will assist the other muscles in extend- 

 ing the wrist and the elbow. 



The EXTENSOR MINIMI DIGITI, E, is the most slender muscle on the back 

 of the forearm, and appears to be but a part of the common extensor. Its 

 origin is in common with that of the extensor communis, but it passes 

 through a distinct sheath of the annular ligament. Beyond the ligament 

 the tendon splits into two, and only one unites by a cross piece with the 

 tendon of the common extensor : both finally join the common expansion 

 on the first phalanx of the little finger. 



Action. It extends the little finger and moves back the wrist and elbow. 

 As the inner piece of the split tendon is not united with the common ex- 

 tensor it can straighten the digit during flexion of the other fingers. 



The EXTENSOR CARPI ULNARis MUSCLE, F, arises from the common 

 tendon, and the aponeurosis of the forearm ; it is also fixed by fascia to 

 the posterior border of the ulna below the anconeus muscle (about the 

 middle third). Its tendon becomes free from fleshy fibres near the annu- 

 lar ligament, and passes through a separate sheath in that structure to be 

 inserted into the base of the metacarpal bone of the little finger. 



Beneath this extensor are some of the muscles of the deep layer, with 

 part of the ulna. On the outer side is the extensor of the little finger, 

 with the posterior interosseous vessels. 



Action. As the name expresses, the muscle puts back the wrist and in- 

 clines the hand towards the ulnar side : it can then extend the elbow joint. 



The ANCONEUS, G, is a small triangular muscle near the elbow. It arises 



