The posterior interosseous, the remaining division of the artery, leaves the front of the limb by passing between the radius and ulna above the superior border of the interosseous membrane, and next becomes visible in the back of the forearm, between the inferior border of the supinator brevis and the ex tensor ossis metacarpi pollicis. In the re mainder of its extent it lies on the muscles which arise from the posterior surface of this membrane, and beneath the more superficial layer of extensors and supinators, until it arrives at the wrist. Here, lying on the pos terior ligament of the joint, it breaks up into its terminal ramifications, which inosculate freely with the posterior carpal arteries of the radial and ulnar, and with the perforating branch of the anterior interosseous division.
While this vessel is passing between the two bones above the ligament, it gives off' the recurrent interosseous branch, which, usually of considerable size, perforates the lower part of the supinator radii brevis to reach the back of the forearm. Subsequently it is directed upwards, lying on this muscle, and beneath the anconeus, until it attains the lower part of the arm, where it terminates by anastomosing with a large branch or branches which proceed from the superior profunda, as it turns round the humerus, and in the sub stance of the outer head of the triceps. It supplies the muscles between which it is situated, and sends a branch to the articula tion of the elbow-joint.
A very constant branch, though usually only of small size, is the twig from the ulnar artery which accompanies the median nerve, con tinuing along it through the forearm until gradually lost from increasing minuteness. It is the basis of an important variety which will be mentioned hereafter.
Low down in the forearm, the ulnar artery gives off a branch which runs along the ulnar side of the metacarpus, and supplies this side of the little finger with a dorsal twig. Ac companied by a branch of the ulnar nerve, it turns backwards from the vessel just above the inferior extremity of the ulna, beneath the flexor carpi ulnaris tendon ; and reaching the inner side of the wrist, continues in a direct line to its termination. It anastomoses with the posterior carpal arch, and, on the metacarpus, with the palmar arch of the ulnar artery.
The remaining branches of the ulnar in the forearm are two, the arterice carpi ulnares anterior et posterior, which occupy a position closely resembling that of the similar branches from the radial artery on the opposite side of the limb. Each runs transversely outwards on its respective surface of the wrist joint, and unites with the radial branch, and from this union, (which, in the case of the posterior vessels, is a " carpal arch " in size and re gularity of arrangement) branches perforate the ligaments to supply the articulations and bones of the carpus. In addition to the
opposite vessel, the anterior inosculates with the tertnination of the anterior interosseous and with small branches sent upwards from the superficial and deep palmar arches : while the arch formed by the posterior joins the posterior interosseous, and the dorsal branch of the anterior interosseous ; and gives off a small branch which occupies each of the two ulnar metacarpal spaces on the back of the hand.
The communicans ulnw is a branch of large size, which passes away from the posterior surface of the ulnar vessel at about the lower border of the annular ligament, and disappears by sinking between the abductor and opponens minimi digiti, to join, deeply in the palm, with the uhlar extremity of the palmaris profunda or palmar arch of the radial, to which it usually approximates in size. It gives small branches to the muscles while passing between them.
In the palm of the hand, the digital arteries are the only branches of the vessel which attain any size : there are four of these, the first supplying the ulnar side of the little finger, and the remaining three corresponding each to the opposed sides of two fingers : the most external being between the middle one and the index. They occupy a situation super ficial to the nerves and tendons, and continue forwards, each as a single branch, until they reach to the clefts of the fingers ; lying in in tervals between the commencing sheaths of the tendons, and limited in front by the trans verse ligament of the fingers, and behind by the strong ligament on the heads of the metacar pal bones. In this space lies also the similar digital branch of the median nerve, but beneath the artery, and at its inferior termination, each bifurcates into branches for the neigh bouring side of the fingers which bound the cleft. Here the vessels are crossed by the nerves, and in the remainder of their length, are situated along the border of the finger, to its termination, the nerve being anterior. At the extremity of the finger, the branch of each side gives off' a twig to the under surface of the nail, and the remainder immediately uniting in an arch with the similar branch of the op posite side, breaks up into a network, whose meshes thus form a highly vascular subtra tum to the sensitive papillary surface which especially occupies this part. In its course along the finger, beside many smaller branches, each digital artery gives off' a transverse branch lust above the several phalangeal articulations ; which, by joining with its fellow, forms an arch whence proceed the smaller vessels to the joint.