Action.—­The action of the perforans is to flex the third on the second, and the second on the first phalanx.  The latter it flexes in turn on the metacarpus.  It also assists in the flexion of the entire foot on the forearm, and in supporting the angle of the metacarpo-phalangeal articulation when the animal is standing.

D. THE ARTERIES.

So far as the arteries supplying the foot are concerned, we shall be interested in following up the distribution of the two digitals, which are the terminal branches of the Large Metacarpal.

THE LARGE METACARPAL, OR COLLATERAL ARTERY OF THE CANNON.—­This, the larger terminal branch of the posterior radial artery, needs brief mention, for the reason that we shall be afterwards concerned with it in the operation of neurectomy.  Its point of origin is the inside of the inferior extremity of the radius.  Descending in company with the flexor tendons, and passing behind the carpus and beneath the carpal sheath, it continues its descent, in company with the internal plantar nerve and the internal metacarpal vein, on the inner side of the flexor tendons until just above the fetlock.  At this point it bifurcates into the digital arteries.

From the carpus downwards the large metacarpal artery, the internal metacarpal vein, and the internal plantar nerve are in close relation with each other.  The vein holds the anterior position.  The artery is between the two, and has the nerve in close contact with it behind.

THE DIGITAL ARTERIES, OR COLLATERAL ARTERIES OF THE DIGIT.—­These are of large volume, and carry the blood to the keratogenous apparatus of the foot.  They separate from each other at an acute angle, and pass over the side of the fetlock, one to the inside, the other to the outside, to reach the internal face of the basilar process of the os pedis, where they bifurcate to form the Plantar and Preplantar arteries.  In the whole of their course the digital arteries follow the flexor tendons, and are related in front to the digital vein, and behind to the posterior branch of the plantar nerve.  This is the nerve implicated in the lower operation of neurectomy, and its relation to adjoining structures will be detailed under Section F. of this chapter.  During its course the digital artery gives off branches in the following positions: 

1. At the Fetlock numerous branches to the metacarpo-phalangeal articulation, the sesamoid sheath, and the tendons.

2. At the Upper Extremity of the First Phalanx branches for the supply of the surrounding tissues, and for the tissues of the ergot.

3. Towards the Middle of the Third Phalanx, the Perpendicular artery of Percival.  This arises at a right angle from the main vessel, and immediately divides into two series of ramifications—­an ascending and a descending.  The ramifications of these series freely anastomose with corresponding vessels of the opposite side.