The =ulnar part= (Fig. 77, _x_) is a conical muscle taking origin from the outer surface of the tendon and muscle of the palmaris longus and from the adjacent ligament. It gives rise to two or three tendons (Fig. 88, _b_) which arise from distinct slips of the muscle; these pass to the ulnar two or three digits. That passing to the fourth or fifth digit receives an accessory slip from a small mass of fibres attached to the transverse ligament on the radial side of the palmaris tendon. The =radial part= is a small flat triangular muscle which takes origin from the outer surface of the tendon formed by the junction of the tendons of the first and second parts of the flexor profundus. It divides into two slips, each giving rise to a tendon. These tendons (Fig. 88, _b′_) pass to the second and third digits. That to the third digit may divide into two, one of which goes to the fourth digit. Each of the four tendons (Fig. 88, _b_ and _b′_) is perforated by a tendon of the flexor profundus (Fig. 88, _c_) as it passes through the fibrous pulley-ring at the base of the first phalanx. It then continues beneath the flexor profundus tendon through the second pulley-ring (2) and is inserted into the base of the second phalanx. _Relations._--Outer surface of the ulnar part with the integument; inner surface with the palmaris longus (Fig. 77, _s_). Outer surface of the radial part with the palmaris longus; inner surface with the flexor profundus. _Action._--Flexor of the second phalanx of digits 2-5. =M. flexor profundus digitorum= (or perforans).--This arises by five heads, the tendons of which join one another at the wrist to make the strongest and deepest of the flexor tendons. The _first_ or _ulnar head_ has _origin_ (Fig. 87, _g_) from the dorsal half of the medial (radial) surface of the ulna from the proximal lip of the semilunar notch to within a centimeter of the styloid process. Its fibres converge to a large flat tendon which forms the lateral (ulnar) and superficial part of the common tendon. [Illustration: FIG. 87.--RADIUS AND ULNA, Medial or Flexor Side, with M. pronator quadratus and the Areas of Attachment of Other Muscles. _a_, M. pronator quadratus; _b_, tendon of M. biceps; _c_, conjoined tendon of M. brachialis and M. clavobrachialis; _d_, insertion of caput longum of M. triceps; _e_, insertion of short portion of caput mediale of M. triceps; _f_, origin of M. flexor carpi ulnaris; _g_, origin of first head of M. flexor profundus digitorum; _h_, origin of fifth head of M. flexor profundus digitorum; _i_, intermediate portion of caput mediale of M. triceps.]