INNERVATION OF THE FOREFOOT
Four nerves attend to the innervation of most of the structures distal to the carpus: the medial and lateral palmar nerves from the median nerve, and the palmar and dorsal branches of the ulnar nerve.
All but the dorsal branch of the ulnar lie palmar to the large metacarpal bone. The medial palmar nerve lies in the groove between the interosseus and the flexor tendons. In midcannon it detaches a communicating branch that crosses obliquely over the superficial flexor tendon (where it is palpable) to join the lateral palmar nerve. A little above the fetlock the medial palmar becomes the medial digital nerve, which immediately gives rise to one or two dorsal branches that ramify over the dorsomedial aspect of the digit and coronet. The main trunk of the digital nerve continues with the like-named artery over the outer aspect of the proximal sesamoid bone, passes under the ligament of the ergot (Figure 23-44), and then disappears into the hoof. The neurovascular bundle may be palpated against the sesamoid bone. Small branches supply the structures caudal to the phalanges. The nerve ends by supplying the laminar and sole dermis.The lateral palmar nerve, it will be recalled, exchanged fibers with the palmar branch of the ulnar nerve at the carpus. It emerges from the short (1- to 2-cm) union and takes a course and has a distribution similar to that of the medial palmar nerve, including the ramifications in the digit. The first branch of this composite nerve arises at the carpus and soon splits into thin medial and lateral palmar metacarpal nerves that descend, deeply embedded, along the axial surface of the splint bones. These nerves supply the interosseus and the palmar pouch of the fetlock joint before becoming subcutaneous at the distal ends of the splint bones. They now supply the dorsal pouch of the joint before mingling with the dorsal branches of the digital nerves; they do not reach the coronet.
All of these nerves can be blocked at various levels— mainly for the diagnosis of lameness. The rationale of the procedure is that a lame horse temporarily becomes sound when the area that contains the undetected lesion is desensitized. A sequence of injections, in which increasingly larger territories are desensitized, is therefore required. Four sites are commonly used.
1. The palmar digital blocks have as their targets the digital nerves, level with the pastern joint and just proximal to the hoof cartilage (the digital artery lies next to the nerve). The block desensitizes all structures in the hoof, except the dorsal part of the coronary band.
2. Blocks at the level of the proximal sesamoid bones have as their targets the digital nerves and their dorsal branches (the digital artery and vein lie dorsal to the nerve adjacent to the dorsal branches). The block desensitizes the digit, except the dorsal aspect of the pastern.
3. In the distal metacarpal block the injections are made level with the distal extremities of the splint bones. The target combines the palmar nerves (the palmar vein lies dorsal to the nerve; the artery lies deep to it) and branches of the palmar metacarpal nerves (subcutaneous, distal to splint bone; deep, between splint bone and interosseus; Figure 23-45). The block desensitizes the digit, including the fetlock joint, with the possible exception of its dorsal pouch.
4. In the proximal metacarpal block the injections are made on the axial surface of the proximal end of the splint bones. The targets are the medial and lateral palmar nerves and the origin of the metacarpal nerves from the latter (large vessels accompany especially the medial palmar nerve). The block desensitizes the digit, including the fetlock joint (with the possible exception of its dorsal pouch), and most structures in the caudal metacarpus; because of dis- topalmar out-pouchings of the nearby carpometacarpal joint, this and the midcarpal joint may also be desensitized.
The autonomous zones of skin innervation are shown in Figure 23-1. A skin prick in the center of a zone tests for the integrity of the particular nerve.