Page 41 - Adams and Stashak's Lameness in Horses, 7th Edition

Functional Anatomy of the Equine Musculoskeletal System 7

from each palmar process of the bone and project proxi­
mad to the coronary band of the hoof where they may
VetBooks.ir their axial surfaces, convex on their abaxial surfaces,
Distal phalanx
be palpated (Figure 1.9). The cartilages are concave on
and thicker distally where they attach to the bone. Each
cartilage is perforated in its palmar half by several
Periosteum foramina for the passage of veins connecting the palmar
venous plexus with the coronary venous plexus.
Five ligaments stabilize each ungual cartilage
(Figure 1.9):
1. A short, prominent ligament extends from the dorsal
surface of the middle phalanx to the dorsal part of
the cartilage.
2. A poorly defined elastic band extends from the side
Solar corium
of the proximal phalanx to the proximal border of
the cartilage and also detaches a branch to the digital
cushion.
3. Several short fibers attach the distal part of the carti­
lage to the distal phalanx.
4. A ligament extends from the dorsal aspect of the car­
Papillae of solar tilage to the termination of the tendon of insertion of
corium covered by the common digital extensor muscle. The dorsal part
solar epidermis of each cartilage also serves as part of the distal
attachment for the respective collateral ligament of
the coffin joint.
5. An extension of the collateral sesamoidean ligament
(CSL) attaches the end of the navicular bone to the
cartilage of the distal phalanx.
Between the cartilages on the palmar side of the foot
is the digital cushion, a highly modified subcutis consist­
Tubular and ing of a meshwork of collagen, elastic fibers, adipose
intertubular horn tissue, and small masses of fibrocartilage (Figure 1.10).
of the sole Only a few blood vessels ramify in the digital cushion.
Dorsoproximally the digital cushion connects with the
distal digital annular ligament. The apex of the wedge‐
shaped digital cushion is attached to the deep digital
flexor tendon (DDFT) as the latter inserts on the solar
surface of the distal phalanx. The base of the digital
cushion bulges into the bulbs of the heels. The digital
cushion serves an anticoncussive function.
As the DDFT courses to its insertion on the distal pha­
lanx, it is bound down by the distal digital annular liga­
ment, a sheet of deep fascia supporting the terminal part
of the tendon and sweeping proximad to attach on each
side of the proximal phalanx (Figure 1.11). The tendon
passes over a complementary fibrocartilaginous plate on
Figure 1.7. Histological relationships of periosteum, corium, and the proximal extremity of the palmar surface of the middle
horn of the sole. phalanx; this is the middle scutum (L. shield), which pro­
vides a smooth gliding surface for the tendon. Then the
tendon gives off two small secondary attachments to the
the coronary band (Figure 1.5). Deep to the coronary distopalmar surface of the second phalanx; these are part
band, the subcutis is modified into the highly elastic cor­ of the so‐called T ligament (Figures 1.10 and 1.12).
onary cushion. The coronary band and cushion form the Continuing toward its primary attachment on the
bulging mass that fits into the coronary sulcus of the flexor surface of the distal phalanx, the DDFT passes over
hoof (Figure 1.4). Part of the coronary venous plexus is the navicular (podotrochlear) bursa, interposed between
within the coronary cushion. The plexus receives blood the tendon and the fibrocartilaginous distal scutum cover­
from the dorsal venous plexus in the laminar corium. ing the flexor surface of the navicular bone. From the
exterior of the foot, the navicular bursa lies deep to the
approximate middle third of the frog on a plane parallel
Internal StructureS of tHe foot
to the coronet over the quarters of the hoof wall.
The “collateral” cartilages of the distal phalanx (often The proximal border of the navicular bone (distal
“lateral cartilages”; most correctly ungual cartilages) lie sesamoid bone) possesses a groove containing foramina
deep to the hoof and the skin, covered on their abaxial for the passage of small vessels and nerves. The distal
surfaces by the coronary venous plexus. They extend border of the bone has a small, elongated facet that