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.
4. At the Superior Border of the Lateral Cartilage, the Artery of the Plantar Cushion. This is directed obliquely downwards and backwards, under cover of the cartilage, and is distributed to the middle portion of the complementary apparatus of the os pedis, as well as to the villous tissue and the coronet. A branch of it is turned forwards to join with the coronary circle in forming the circumflex artery of the coronet.
Fig. 14. - The Arteries Of The Foot. The Digital; 2, The Perpendicular - (A) Its Ascending Branch, (B) Its Descending Branch; 3, Circumflex Artery Of Coronary Cushion; 4, The Preplantar (Ungual) Artery - This Is Seen Issuing From The Preplantar Foramen, And Distributing Numerous Ascending (C) And Descending (D) Branches (The Latter Concur In Forming The Circumflex Artery Of The Toe); 5, The Circumflex Artery Of The Toe; 6, At The Point Marked (*) The Terminal Branch Of The Digital - Namely, The Plantar Ungual - Is Hidden Behind The Lateral Cartilage; 7, The Lateral Cartilage.
5. Under the Lateral Cartilage two transverse branches, an anterior and a posterior, to form the Coronary Circle. The numerous ramifications of these branches anastomose both anteriorly and posteriorly with their corresponding branches of the artery of the opposite side. This circle closely embraces the os coronae. Among the larger branches given off from its anterior portion are two descending, one on each side of the extensor pedis, to assist in the formation of the Circumflex Artery of the Coronary Cushion. The formation of this last-named artery is completed posteriorly by the before-mentioned branch from the artery of the plantar cushion.
The Preplantar (Ungual[A]) Artery. - This, the smaller of the two terminal branches of the digital, is situated inside the basilar process of the os pedis. It turns round this to gain the fissure between the basilar and retrossal processes, and becomes lodged in the preplantar fissure. Here it terminates in several divisions which bury themselves in the os pedis. Before leaving the inner aspect of the pedal wing it supplies a deep branch to the heel and the villous tissue. Gaining the outer aspect of the wing, it distributes a further backward branch, which passes behind the circumflex artery of the pedal bone, and, during its passage in the preplantar fissure, gives off ascending and descending branches, which ramify in the laminal tissue.
The Plantar (Ungual[A]) Artery. - This, the larger of the two terminals of the digital, may be looked upon as a continuation of the main vessel. Running along the plantar groove, it gains the plantar foramen. Here it enters the interior of the bone (the semilunar sinus) and anastomoses with the corresponding artery of the opposite side. The circle of vessels so formed is called the Plantar Arch or the Semilunar Anastomosis.
[Footnote A: The epithet 'ungual' is added by Chauveau to distinguish these arteries from the properly so-called plantar arteries - the terminal divisions of the posterior tibial artery.]
From the semilunar anastomosis radiate two main groups of arterial branches, an ascending group and a descending one. The ascending branches penetrate the substance of the os pedis, and emerge by the numerous foraminae on its laminal surface. The descending branches, larger in size, also penetrate the substance of the pedal bone, and emerge in turn from the foraminae cribbling its outer surface - in this case the set of larger foraminae opening on its inferior edge. Having gained exit from the bone, their frequent anastomosis, right and left, with their fellows forms a large vessel following the contour of the inferior edge of the os pedis. This constitutes the Circumflex Artery of the Toe.