6. The =palmar arch= (Fig. 124) is formed by the termination of the radial artery (_a_) in the palm and its junction with the end of the ulnar (_f_). The radial artery reaches the palm between the bases of the second and third metacarpals and passes thence toward the ulnar side and distad, piercing the interosseus muscle of the third digit and lying on the outer surface of the interossei of the third and fourth digits beneath the adductors of the second and fifth digits. A small communicating branch from the radial passes to it between the first and second metacarpals. Branches of the palmar arch: =A. princeps pollicis et indicis= (_c_) leaves the palmar arch near its radial end and sends a branch onto the ulnar side of the thumb and one onto the radial side of the index. The =palmar interosseæ= (_d_) are three in number. They leave the palmar arch (_b_) and pass distad and dorsad in the intervals between the four ulnar digits. The radial one passes along the ulnar side of the first digit, the ulnar one along the radial side of the fifth digit. The middle one divides and supplies the contiguous sides of the third and fourth digits. Each of these interosseous arteries sends off muscular branches (_e_) to the short muscles in the palm, and branches into the fibrous pad which occupies the palm of the hand. The palmar arch also sends branches onto the dorsum of the hand at the sides of the fourth metacarpal. Branches pass from these proximad to the wrist. B. AORTA ABDOMINALIS. THE ABDOMINAL AORTA.--The abdominal aorta emerges into the abdomen from between the crura of the diaphragm, at about the level of the second lumbar vertebra. It passes caudad along the dorsal middle line, lying to the left of the inferior vena cava. It gives off =parietal= branches to the body wall, and =visceral= branches to the viscera, and ventrad of the first sacral vertebra it gives off two large branches on each side, the =external iliac= (Fig. 126, _k_) and the =hypogastric= (Fig. 126, _l_)--a very small median vessel, the =sacralis media= (Fig. 126, _o_), continuing the course of the aorta and passing into the tail. The aorta gives off the following branches: =A. cœliaca=; =A. mesenterica superior=; =Aa. adrenolumbales=; =Aa. renales=; A. =mesenterica inferior=; =Aa. iliolumbales=; =Aa. lumbales= (seven pairs); =Aa. iliacæ externæ=; =Aa. hypogastricæ=.