220 MUSCLES OF THE UPPER LIMB. 



inserted into an expansion of the extensor tendon on the dorsal aspect 

 of the metacarpal phalanx of the finger. The two inner lumbricales 

 muscles take origin likewise from the ulnar margins of the second and third 

 tendons. 



The number of these muscles is not imfrequently diminished to three, and in rare 

 instances is increased to five or six. The destination of one or two of them is often 

 changed, and one finger (most frequently the third or fourth) has sometimes two 

 inserted into it. Lastly, one muscle may be inserted into two fingers. 



The flexor longus pollicis, placed side by side with the flexor profundus, 

 arises from the grooved surface, on the fore part of the radius from the 

 oblique line to the edge of the pronator quadratus ; it arises also from 

 the adjacent part of the interosseous ligament, and by a rounded fleshy 

 and tendinous slip distinct from the rest of the muscle, from the inner part 

 of the coronoid process. The muscle ends in a tendon which passes behind 

 the annular ligament of the wrist close to the trapezium, turns outwards 

 between the two heads of the flexor brevis and between the sesamoid bones, 

 and, entering a canal similar to those of the other flexor tendons, is finally 

 inserted into the base of the second phalanx of the thumb. 



Inferiorly, a part of this muscle is placed superficially between the flexor carpi 

 radialis and the supinator longus, where it supports the radial vessels. 



The pronator quadratus, placed close to the bones behind the last two 

 muscles, arises from the anterior and inner surfaces of the ulna for about 

 two inches above its carpal extremity ; its fibres cross the lower part of 

 the forearm, some transversely and others obliquely, and they are inserted 

 for an equal distance into the fore part of the radius. 



6. Posterior and Outer Regions (Extensors and Supinators). 



The muscles of this group are, like those of the front of the forearm, 

 divided into a superficial and a deep layer. 



The superficial muscles are six in number, viz., the supinator longus, the 

 extensores carpi radiales longior and brevier, the extensor communis 

 digitorum, extensor minimi digiti, and extensor carpi ulnaris. 



The supinator radii longus arises from the upper two-thirds of the 

 external condyloid ridge of the humerus, and is interposed between the 

 brachialis anticus and the external intermuscular septum, to which its fibres 

 are attached. The thin fleshy mass proceeding from this elongated origin 

 descends upon the anterior and outer border of the forearm, and ends 

 about the middle in a flat tendon, which is inserted into the external border 

 of the radius, near the base of the styloid process. 



This muscle is covered only by skin and fascia, except at its insertion, where two 

 of the extensor tendons of the thumb lie superficial to it. 



The extensor carpi radialis longior, partly covered by the supinator longus, 

 arises from the lower part of the external condyloid ridge of the humerus, 

 and from the intermuscular septum. Its muscular belly ends at the 

 meeting of the upper and middle thirds of the forearm in a flat tendon, 

 which passes conjointly 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 bursa lies beneath the tendon at its insertion. 



