The central portion lies between the lateral cartilages and immediately under the coronary cushion.  The lateral portions are ramifications on both surfaces of the lateral cartilages.  The ramifications on the lateral cartilages may be again distinguished as superficial and deep.  The superficial layer is distributed over the external face of the cartilage, forming thereon a dense network, and finally converges towards the superior limit of the plexus to form ten or twelve principal branches, which again unite to form two large vessels.  These vessels, by their final fusion at the lower end of the first phalanx, constitute the digital vein.  The deep layer is formed, as before described, by ascending branches from the posterior parts of the podophyllous and solar plexuses, and by branches from the intra-osseous system of the pedal bone.  The veins of this deep layer finally drain into the two vessels proceeding from the superficial layer, which go to the formation of the digital vein.

THE DIGITAL VEINS—­These arise from the network formed on the surfaces of the lateral cartilages, and ascend in front of the digital arteries to unite above the fetlock, where they form an arch between the deep flexor and the suspensory ligament.  From this arch (named the Sesamoidean) proceed the Metacarpal Veins.

THE METACARPAL VEINS.—­Three in number, they are distinguished as an Internal and an External Metacarpal, and a Deep or Interosseous Metacarpal.  As we shall be concerned with these in the higher operation of neurectomy, we may give them brief mention.

THE INTERNAL METACARPAL VEIN, the largest of the three, has relations with the internal metacarpal artery and the internal plantar nerve.  These relations were shortly discussed under the section devoted to the arteries, to which the reader may refer.

THE EXTERNAL METACARPAL VEIN.—­This ascends on the external side of the flexor tendons in company with the external plantar nerve.

The Interosseous Vein.—­This is an irregular vessel running up between the suspensory ligament and the posterior face of the large metacarpal bone.

F. THE NERVES.

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.