THE UPPER LIMB 361 



of the ulnar. It is directed backwards to the upper border of the 

 interosseous membrane, where it divides into the anterior and 

 posterior interosseous arteries. 



(i) The anterior interosseous artery descends on the front of the 

 interosseous membrane, having a vena comes on either side of it, 

 and the anterior interosseous nerve on its outer side. It lies 

 between the flexor longus pollicis externally and the flexor profundus 

 digitorum internally, both of which overlap it, and at the upper 

 border of the pronator quadratus it divides into two terminal 

 branches, anterior and posterior. 



Branches. — ^These are as follows : the median artery (comes 

 nervi mediani) is a long, slender branch which arises from the 

 commencement of the vessel. It at once gets in contact with the 

 median nerve, which it accompanies beneath the flexor sublimis 

 digitorum, supplying the nerve and that muscle. If of large size, 

 it passes beneath, or over, the anterior annular ligament into the 

 palm, where it may join the superficial palmar arch, or if that 

 should be abnormal, it furnishes certain digital arteries. Muscular 

 branches are given off to the deep layer of muscles, and to the 

 extensor muscles of the thumb on the back of the interosseous 

 membrane. The branches to the latter muscles pierce the mem- 

 brane. The nutrient or medullary arteries enter the radius and 

 ulna. The anterior terminal branch descends beneath the pro- 

 nator quadratus and joins the anterior carpal arch. The posterior 

 terminal branch, which, passing backwards through the interosseous 

 membrane, and having anastomosed with the posterior interosseous, 

 descends beneath the extensor tendons and the posterior annular 

 ligament to the back of the wrist, where it joins the posterior 

 carpal arch. 



(2) The posterior interosseous artery passes backwards between 

 the upper border of the interosseous membrane and the oblique 

 ligament. At the back of the forearm it appears between the 

 supinator radii brevis and extensor ossis metacarpi pollicis, where 

 it is joined by the posterior interosseous nerve. The artery, with 

 the nerve, then descends between the superficial and deep group 

 of muscles until it reaches the upper border of the extensor longus 

 pollicis. Here the artery leaves the nerve, and passes over that 

 muscle and the extensor indicis. At the lower border of the 

 latter muscle it anastomoses with the posterior terminal branch 

 of the anterior interosseous, and then ends in articular branches 

 to the wrist- joint, though it may pass to join the posterior carpal 

 arch. 



Branches. — ^These are as follows : the posterior interosseous re- 

 current arises as soon as the vessel reaches the back of the forearm. 

 It passes upwards beneath the anconeus to the back of the external 

 epicondyle, where it anastomoses with the posterior branch of the 

 superior profunda of the brachial. It also gives twigs over the 

 back of the olecranon process which anastomose with branches 

 of the posterior ulnar recurrent, and so form the olecranon rete. 



