The =internal= and =external digital arteries= (A. digitalis medialis,
lateralis) are formed by the bifurcation of the common digital at the
distal third of the metacarpus. They diverge, pass down over the outer
surface of the corresponding sesamoid at the fetlock, and descend
parallel with the borders of the deep flexor tendon to the volar grooves
and foramina of the third phalanx. Entering the latter the two arteries
unite in the semilunar canal and form the =terminal arch= (Arcus
terminalis), from which numerous branches pass through the bone to the
wall surface and ramify in the matrix of the wall and sole of the hoof.
A number of branches emerge through the foramina at the lower border,
where they anastomose with each other in arciform fashion. These
branches were named by Spooner the inferior communicating arteries, and
the anastomotic arch formed by them is termed the circumflex artery of
the third phalanx (Chauveau) or the artery of the lower border of the
third phalanx (Leisering).
Each artery is accompanied by a vein and by the digital nerves. Above
the fetlock the artery is most deeply placed and is covered by the vein;
the nerve is behind the vein. At the fetlock the artery has become
superficial and is related to the vein in front and the posterior branch
of the nerve behind. The anterior branch of the nerve crosses over the
artery obliquely to the lateral aspect of the first phalanx. The vessels
and nerves are crossed obliquely by a small tendinous band, the ligament
of the ergot, which begins in the fibrous basis of the ergot at the back
of the fetlock and extends downward and forward to end in the fascia at
the side of the pastern joint.
In addition to branches to the joints, tendons and synovial sheath,
ergot, and skin, the digital arteries give off:
1. The =artery of the first phalanx= or perpendicular artery (A.
phalangis primæ), a short trunk which arises at a right angle about the
middle of the first phalanx, and divides into dorsal and volar branches
(Rami dorsales et volares). The dorsal (anterior) branch passes between
the first phalanx and the extensor tendon and ramifies on the front of
the digit, anastomosing with its fellow. The volar (posterior) branch
dips in between the flexor tendons and the inferior sesamoidean
ligaments and anastomoses with the opposite artery.
2. The =artery of the plantar cushion= (A. toricæ phalangis tertiæ)
arises at the proximal border of the lateral cartilage and passes
backward and downward to ramify in the plantar cushion and the matrix of
the heels and frog.
3. The =dorsal artery of the second phalanx= (Ramus dorsalis phalangis
secundæ) arises a little above the navicular bone, and passes forward
under cover of the lateral cartilage and the extensor tendon to the
front of the second phalanx, where it anastomoses with the opposite
vessel. It gives branches to the skin, the tendon, the coffin joint, and
the coronary matrix of the hoof.
4. The =volar artery of the second phalanx= (Ramus volaris phalangis
secundæ) is smaller than the preceding, opposite to which it arises. It
passes above the proximal border of the third sesamoid and unites with
the opposite artery.
The arteries of the second phalanx form what is termed by Chauveau the
=coronary circle=. The dorsal (anterior) part of the circle gives off
commonly an artery (A. coronalis phalangis tertiæ) near either side of
the extensor tendon, which divides into two branches. The central
branch unites with that of the opposite side, while the other joins a
branch of the artery of the plantar cushion. In this way is formed the
=circumflex artery of the coronary cushion=, an anastomotic arch which
lies on the extensor tendon at the coronet. In some cases descending
branches of the arteries of the first phalanx concur in the formation
of the arch.
5. The =dorsal artery of the third phalanx= or preplantar artery (A.
dorsalis phalangis tertiæ) arises at the deep face of the wing of the
third phalanx, passes outward through the notch or foramen there, and
runs forward in the groove on the wall surface. It gives off ascending
and descending branches, which ramify in the matrix of the wall of the
hoof, anastomosing above with the circumflex artery of the coronary
cushion and below with the circumflex artery of the third phalanx.
Before passing through the wing it detaches a retrograde branch to the
plantar cushion, and after emerging one which ramifies on the outer
aspect of the lateral cartilage.
The terminal part of the digital artery after giving off the preceding
vessel is sometimes termed the plantar.