The Plantar Nerves. - These are two in number, and are distinguished as Internal and External.
The Internal Plantar Nerve lies behind and in close contact with the great metacarpal artery during that vessel's course down the region of the cannon. A point of interest is that it gives off at about the middle of the cannon a branch which bends obliquely downwards and behind the flexor tendons to join its fellow of the opposite side - namely, the external plantar. This it joins an inch or more above the bottom of the splint bone. Measured in a straight line, this is about 2-1/2 inches below its point of origin. Near the fetlock, at the level of the sesamoids, the internal plantar nerve ends in several digital branches.
The External Plantar Nerve. - This holds a position to the outside of the metacarpal region, analogous to that of the internal plantar nerve on the inside of the limb, running down on the external edge of the flexor tendons. Unlike the internal nerve, it is accompanied by a single vessel only, the external metacarpal vein, behind which it lies. At the level of the sesamoid bones it divides, as does the internal nerve, into three main branches - the digital nerves.
Fig. 15. - The Veins And Nerves Of The Foot. 1, The Digital Vein; 2, Its Main Tributaries, Draining The Podophyllous Plexus, And Concurring To Form The Digital; 3. The Digital Artery (The Main Trunk Only Of This Is Shown, In Order To Show Its Relationship With The Vein And Nerve); 4, The Plantar Nerve, With Its Three Branches - (A) The Anterior Digital, (B) The Middle Digital, (C) The Posterior Digital; 5, The Podophyllous Plexus; 6, Superficial Portion Of The Coronary Plexus; 7, The Peripheral Or Circumflex Vein Of The Toe.
The Digital Nerves. - These are distinguished as Anterior, Middle, and Posterior.
The Anterior Branch descends in front of the vein, distributing cutaneous branches to the front of the digit, and terminating in the coronary cushion.
The Middle Branch descends between the artery and the vein, and freely anastomoses with the two other branches. It terminates in the coronary cushion and the sensitive laminae.
The Posterior Branch. - This is the largest of the three, and may be regarded as the direct continuation of the plantar. At the fetlock it is placed immediately above the digital artery, but afterwards takes up a position directly behind that vessel. Together with the digital artery it descends to near the basilar process of the os pedis. Here it passes with the plantar artery into the interior of the os pedis, and continues its main branch, with the preplantar artery, in the fissure of the same name, to finally furnish supply to the os pedis and the sensitive laminae. It is this nerve which is divided in the low operation of neurectomy.
Beyond the fact of this branch descending, in the region of the pastern, 1 inch behind the digital artery, a further point of interest presents itself to the surgeon, and one to which attention must be paid. This is the presence in close proximity to the nerve of the Ligament of the Pad (Percival), or the Ligament of the Ergot (McFadyean). This is a subcutaneous glistening cord originating in the ergot of the fetlock, passing in an oblique direction downwards and forwards, and crossing over on its way both the digital artery and the posterior branch of the digital nerve.
In the foregoing description of the anatomy, we have taken the fore-limb as our guide. In the hind-limb, where they reach the foot, the counterparts of the tendons, arteries, veins, and nerves differ in no great essential from their fellows in the fore. They will therefore need no special mention.