The =radius= is much the larger of the two bones of the forearm in the
horse. It extends in a vertical direction from the elbow, where it
articulates with the humerus, to the carpus below. It is gently curved,
the convexity being anterior. It consists of a shaft and two
extremities.
The =shaft= (Corpus radii) is curved in its length, somewhat flattened
from before backward, and expanded at its ends. It presents for
description two surfaces and two borders. The =anterior surface= (Facies
dorsalis) is smooth, slightly convex in its length, and rounded from
side to side. The =posterior surface= (Facies volaris) is
correspondingly concave in its length and is flattened in the transverse
direction. At its upper part there is a smooth shallow groove, which
concurs with the ulna in the formation of the =interosseous space= of
the forearm; the nutrient foramen is in the lower part of this groove.
Below this there is in the young subject a narrow, rough, triangular
area to which the ulna is attached by an interosseous ligament; in the
adult the two bones are fused here. A variable rough elevation below the
middle of the surface and close to the internal border gives attachment
to the superior check ligament. The =internal border= (Margo medialis)
is slightly concave in its length and is largely subcutaneous; at its
proximal end there is a smooth area on which the tendon of insertion of
the brachialis muscle lies, and a small rough area just below gives
attachment to that muscle and the long internal lateral ligament of the
elbow joint. The =external border= (Margo lateralis) is more strongly
curved, but presents no special features.
The =proximal extremity= or =head= (Capitulum radii) is flattened from
before backward and wide transversely. It presents an articular surface
(Fovea capituli) which corresponds to that on the distal end of the
humerus; it is crossed by a central sagittal ridge, which has a synovial
fossa on its posterior part, and ends in front at a prominent lip, the
=coronoid process= (Processus coronoideus). Just below the posterior
border there are two concave facets for articulation with the ulna, and
between these and the interosseous space is a quadrilateral rough area
at which the two bones are united by an interosseous ligament. At the
inner side of the anterior surface is the =bicipital tuberosity=
(Tuberositas radii), into which the biceps tendon is inserted. The
=internal tuberosity= is continuous with the preceding eminence, and
furnishes attachment to the short part of the internal lateral ligament.
The =external tuberosity= is more salient; it gives attachment to the
external lateral ligament and to the anterior and lateral extensor
muscles of the digit.
[Illustration:
FIG. 48.—LEFT RADIUS AND ULNA OF HORSE, EXTERNAL VIEW. (After
Schmaltz, Atlas d. Anat. d. Pferdes.)
]
[Illustration:
FIG. 49.—LEFT RADIUS AND ULNA OF HORSE, POSTERIOR VIEW. (After
Schmaltz, Atlas d. Anat. d. Pferdes.)
]
The =distal extremity= is also compressed from before backward. It
presents the =carpal articular surface= (Facies articularis carpea)
which consists of three parts. The inner facet is the largest, is
quadrilateral, concavo-convex from before backward, and articulates with
the radial carpal bone (or scaphoid); the middle one is somewhat similar
in form but smaller, and articulates with the intermediate carpal bone
(or semilunar); the outer facet is smaller, is convex, and articulates
below with the ulnar carpal (or cuneiform) and behind with the accessory
carpal (or pisiform). The anterior surface presents three =grooves=,
separated by ridges. The middle one is vertical and gives passage to the
tendon of the extensor carpi radialis; the outer one is similar and
contains the tendon of the anterior extensor of the digit; the inner one
is small and oblique and lodges the tendon of the extensor carpi
obliquus. The posterior aspect is crossed by a rough ridge, below which
are three depressions. On either side is a tuberosity (Tuberculum
ligamenti) to which the lateral ligament is attached. The outer one is
marked by a small vertical groove for the passage of the lateral
extensor tendon.
=Development.=—The radius ossifies from four centers, viz., one each for
the shaft, the two extremities, and the outer part of the distal end;
the last is morphologically the distal end of the ulna which has fused
with the radius, and the line of fusion is often indicated by a distinct
groove on the carpal articular surface. The proximal extremity unites
with the shaft at about one and a half years, the distal end at about
three and a half years.
[Illustration:
FIG. 50.—UPPER HALF OF RADIUS AND ULNA OF HORSE, INTERNAL VIEW. (After
Schmaltz, Atlas d. Anat. d. Pferdes.)
]