The flexor sublimis digitorum - named from its action as a flexor of the fingers, and from its position in relation to the deep flexor (sublimis = superficial) - is a fusiform sheet, with two heads above, and dividing into four tendons below.

Origin

The first head arises from

• the front of the internal condyle by the common tendon ;
• the intermuscular septum which separates it from the muscles of the first sheet ;
• the internal lateral ligament ;
• and a tubercle at ; the upper part of the inner border of the coronoid process of the ulna.

The second head arises from the oblique line on the anterior surface of the radius.

Insertion

By four tendons into the middle of the sides of the second phalanges of the four fingers.

Structure

The first head arises by short tendinous fibres from the humerus j and ulna, and from the ligament between them. The second head, which is much ! smaller, by fleshy fibres which form a thin sheet covering a part of the flexor longus ; pollicis. Between these two heads the median nerve and the ulnar artery are placed. Converging from these two heads, the fleshy fibres occupy almost the whole breadth i of the forearm, but soon divide into a superficial and deep plane. From the former, which contains the radial head and the more superficial fibres of the first head, the tendons to the middle and ring fingers are derived. The tendon to the middle finger receives the greater part of the radial head in penniform fashion, becoming free close to the anterior annular ligament. That to the ring finger separates high up, and is soon free from fleshy fibres. The deeper plane of the muscle is crossed by a strong tendinous intersection soon after its origin from the first head. It then gives off a fleshy band to join that part of the superficial plane which goes to the ring finger, and afterwards bifurcates to form the tendons for the index and little fingers. I am indebted to Professor Thane for calling my attention to this arrange- ment. Beneath the anterior annular ligament the tendons of the superficial plane, viz. those going to the middle and ring fingers, lie in front of the other two tendons. Here they are invested by the synovial sheath or great loalmar bursa, which is common to them and the other tendons which pass through this space, and which extends to about the middle of the palm. At the heads of the metacarpal bones the tendons enter the vaginal sheaths of the flexors of the fingers, and each tendon becomes concave behind to correspond with the convexity of the tendon of the deep flexor upon which it now rests. At the middle of the first phalanx the tendon splits, and the halves separate to allow the passage of the tendon of the flexor profundus. The two halves again unite opposite the base of the second phalanx in such a way that the parts now in contact are a direct continuation of what were before the borders of the tendon; while the parts of the tendon which correspond to its mesial line above are now most widely separated. After a contact of about a quarter of an inch, the halves of the tendon again separate in order to be attached to the sides of the shaft of the second phalanx.

Nerve-supply

From the outer and inner cords of the brachial plexus (through the seventh and eighth cervical and first thoracic nerves) by branches from the median nerve which enter the deep surface of the muscle at its upper part.

Action

To flex the second phalanges of the four fingers. Being inserted very obliquely, it acts under a considerable mechanical disadvantage, but at the ame time the speed and range of movement corresponding to a slight contraction of the muscle are very great. The size of the angle which it makes with the shaft of the phalanx at its point of insertion is somewhat increased by the raising of its endon from the palmar aspect of the first phalanx by the tendon of the deep flexor, ifter bending the second phalanx of the finger, it will bend also the metacarpo- halangeal joint ; then the three joints which together produce flexion of the wrist ; and finally it will feebly assist in the flexion of the elbow joint.

Front of the forearm: second layer of muscles.

Relations

Superficially, the four muscles of the first sheet and the radial essels and nerves ; deeply, the flexor longus pollicis, flexor profundus digitorum, ,nd pronator quadratus, the ulnar artery and vein, and the median nerve. In the hand, it lies beneath the anterior annular ligament, the palmar fascia, and the luperficial palmar arch, and upon the tendons of the flexor profundus digitorum with the lumbricales.

Variations

The flexor snblimis digitorum varies very little in its origin. It has been leen to arise partly from the pronator radii teres. Occasionally its tendon to the little jinger fails, and the place of this tendon may be taken by a linnbricalis or a special slip from he flexor profundus. Sometimes accessory heads join the tendons in the hand from the lexor profundus or the annular ligament.