4. =Aa interosseæ= (=anterior= (_l_) and =posterior=) are given off usually separately between the origin of the ulnar artery and the biceps tendon. Sometimes they arise as a common trunk which soon divides. The =posterior interosseous artery= passes distad, supplying the flexor muscles of the forearm. It also sends a branch to the pronator teres. The =anterior interosseous= (_l_) may arise from the ulnar artery. It passes dorsad to the interosseous membrane, on which it runs distad; sends a nutrient branch to the ulna, and then passes to the wrist in the substance of the pronator quadratus, which it supplies. It sends a branch to the dorsal surface of the wrist; this ramifies on the carpus, forming a network of small arteries. [Illustration: FIG. 123.--NERVES AND ARTERIES OF THE FOREARM, SEEN FROM THE FLEXOR SIDE. Mm. palmaris longus, flexor carpi radialis, flexor carpi ulnaris, and the third and fourth heads of flexor profundus digitorum have been removed; also part of M. pronator teres. 1, biceps; 2, conjoined tendon of brachialis and clavobrachialis; 3, short portion of caput mediale of triceps brachii; 4, intermediate portion of caput mediale of triceps brachii; 5, cut ends of pronator teres; 6, brachioradialis; 7, extensor carpi radialis longus and brevis; 8, fifth head of flexor profundus digitorum; 9, second head of flexor profundus digitorum; 10, radial part of flexor sublimis digitorum; 11, common tendon of flexor profundus digitorum; 12, first head of flexor profundus digitorum; 13, cut origin of flexor carpi ulnaris. _a_, ulnar nerve; _b_, median nerve; _c_, dorsal cutaneous branch of the ulnar nerve; _d_, palmar branch of ulnar nerve; _e_, deep palmar branch; _f_, superficial palmar branch; _g′_, A. brachialis; _g_, A. radialis; _h_, A. collateralis ulnaris superior; _i_, A. radialis recurrens; _j_, A. ulnaris recurrens; _k_, A. ulnaris; _l_, A. interossea anterior; _m_, large branch of A. radialis, joining A. ulnaris.] 5. =A. ulnaris= (_k_).--The ulnar artery passes beneath the second, third, and fourth parts of the flexor profundus digitorum (but outside of the origin of the fifth part), to the inner surface of the flexor carpi ulnaris. It supplies the flexor carpi ulnaris, the flexor profundus and palmaris longus, and passes on the inner surface of the flexor carpi ulnaris to the wrist. Near the wrist it sends a branch onto the side of the forearm, and another to its midventral part, and ends in a small branch to the wrist on the radial side of the pisiform bone. This branch anastomoses with the radial to form the palmar arch, described below.