_A_, Radius; _B_, ulna; _a_, groove for tendon of extensor carpi
radialis: _b_, groove for common extensor tendon; _c_, proximal
articular surface of radius; _d_, olecranon; _e_, beak (proc.
anconeus) of ulna; _f_, semilunar notch; _g_, coronoid process; _h_,
facet for radius; _i_, groove for lateral extensor tendon; _k_,
groove for tendon of extensor carpi obliquus. (Ellenberger-Baum,
Anat. d. Hundes.)
]
The two bones of the forearm are relatively long and articulate with
each other at either end in such a manner as to allow of slight
movement. A narrow interosseous space separates their shafts. The
=radius= is flattened from before backward and increases in size from
above downward. The shaft forms two curves; one of these, an anterior
convexity, involves the whole shaft; the other, an inner convexity,
affects the upper part. The anterior surface is convex in both
directions and is marked by a groove for the oblique extensor of the
carpus. The posterior surface presents the nutrient foramen in its upper
third, and bears a rough line (Crista interossea) externally for the
attachment of the interosseous ligament. The proximal end (Capitulum
radii) is relatively small and is supported by a distinct neck (Collum
radii). It bears a concave surface (Fovea capituli) above for
articulation with the humerus, and a convex marginal area
(Circumferentia articularis) behind for the ulna. The bicipital
tuberosity is small. There is a large external tuberosity and below this
a rough eminence. The distal extremity is much wider. It has an
extensive concave carpal articular surface. Its inner border projects
downward, forming the =styloid process= of the radius. Externally there
is a concave facet (Incisura ulnaris radii) for the ulna. In front are
three distinct grooves for the extensor tendons. The =ulna= is well
developed, but diminishes in size from above downward. It crosses the
posterior surface of the radius from within outward. The shaft is large
and three-sided in its upper two-thirds, smaller and more rounded below.
Its anterior surface is in general rough. The nutrient foramen is near
the proximal end. A vascular groove descends from it and indicates the
course of the interosseous artery. The proximal end is relatively short.
It is concave and smooth internally, convex and rough externally. The
olecranon is grooved and bears three prominences, of which the posterior
one is large and rounded. The semilunar notch is wide below and
completes the surface for articulation with the trochlea of the humerus.
Below it is a concave surface (Incisura radialis) which articulates with
the back of the head of the radius, and below this is a fossa, which
receives a tuberosity of the radius. The distal end (Capitulum ulnæ) is
small and is produced to a blunt point (Processus styloideus ulnæ). It
articulates with the ulnar carpal below, and has a convex facet on its
antero-internal aspect for the radius.
[Illustration:
FIG. 132.—SKELETON OF DISTAL PART OF THORACIC LIMB OF DOG, EXTERNAL
VIEW.
_4_, Radial + intermediate carpal bone; _5_, ulnar carpal; _6_,
accessory carpal; _7_, second carpal; _8_, third carpal; _9_, fourth
carpal; _10–14_, first to fifth metacarpals; _15_, first phalanx;
_16_, second phalanx; _17_, third phalanx. (After Leisering’s
Atlas.)
]
The =carpus= comprises seven bones, three in the proximal row and four
in the distal. The numerical reduction in the upper row is apparently
due to the fusion of the radial and intermediate, constituting a large
bone which articulates with almost all of the distal surface of the
radius and with the bones of the lower row. It projects prominently on
the posterior surface of the carpus. The ulnar carpal is long; it
articulates with the radius and ulna above and the accessory behind;
below it rests on the fourth carpal and is prolonged downward to
articulate with the fifth metacarpal also. The accessory is cylindrical,
constricted in its middle and enlarged at each end; the anterior
extremity articulates with the ulna and ulnar carpal bone. The first
carpal is the smallest bone of the lower row; it articulates with the
second carpal externally and the first metacarpal distally. The second
carpal is wedge-shaped, the base being posterior; its upper surface is
convex, and its lower is concave and rests on the second metacarpal. The
third carpal is somewhat like the second; its lower surface is concave
and articulates chiefly with the third metacarpal. The fourth carpal is
the largest of the row; it articulates with the fourth and fifth
metacarpals below. Two small bones or cartilages may be found at the
junction of the two rows behind, and a third small bone articulates with
the inner side of the radiointermediate.[27]
Five =metacarpal bones= are present. The first is much the shortest; the
third and fourth are the longest, and are about one-fifth longer than
the second and fifth. The fifth is the widest at the proximal end and is
slightly shorter than the second. They are close together above, but
diverge somewhat below; the first is separated from the second by a
considerable interosseous space. They are so arranged as to form a
convex dorsal surface, and a concave volar surface, which corresponds to
the hollow of the palm of the hand in man. Each consists of a shaft and
two extremities. The shaft is compressed from before backward. In the
third and fourth it is almost four-sided, in the second and fifth
three-sided, in the first rounded. The proximal ends (Bases) articulate
with each other and with the corresponding carpal bones. The carpal
articular surface formed by them is concave from side to side, convex
from before backward. The distal ends (Capitula) have articular surfaces
of the nature of a head, but bear a sagittal ridge behind, except the
first, which is grooved.
[Illustration: