_I_, radius; _II_, ulna. _a_, head of radius; _b_, neck; _c_,
bicipital tuberosity; _d_, articular circumference; _e_, facet for
radial fossa of humerus; _f_, styloid process of radius; _g_, facet
for articulation of scapholunar bone; _h_, semilunar (or great
sigmoid) notch of ulna; _i_, coronoid process; _j_, olecranon; _k_,
area for insertion of brachialis and clavobrachial muscles; _l_, rough
area for attachment of interosseous membrane; _m_, styloid process of
ulna.]
The dorsal surface of the distal end is marked by longitudinal grooves
for tendons, and its lateral or ulnar surface bears a concave facet for
articulation with the ulna.
=Ulna= (_II_, Figs. 49 and 50).--The ulna is a long slender bone,
flattened mediolaterad. It is enlarged at its proximal end and becomes
gradually smaller toward the distal end.
The proximal end is marked ventrally by a deep excavation, the
=semilunar notch=, or =great sigmoid cavity= (_h_). By the saddle-shaped
articular surface of the semilunar notch it articulates with the
trochlea. This articular surface is divided into two parts by a
transverse non-articular area. The distal boundary of the semilunar
notch is a blunt process, the =coronoid= process (_i_), which bears on
its lateral surface a concave facet, the =radial notch=, for the head of
the radius.
The portion of the bone proximad of the semilunar notch is called the
=olecranon= (_j_). It fits into the olecranon fossa of the humerus when
the arm is straightened, and is rough at its end for the insertion of
tendons.
The body of the ulna becomes triangular distad. The distal end is
slightly larger than the shaft just proximad of it, and bears on its
radioventral side a hemispherical head for articulation with the radius.
Distad of the head the bone continues as the flattened =styloid= process
(_m_), which projects distad from its dorsolateral side and is smooth on
the medial side of its apex, for articulation with the cuneiform bone of
the wrist.
=Carpus= (Fig. 51).--The carpus (wrist) consists of seven bones arranged
in two rows, three in the proximal row and four in the distal row.
Beginning on the medial side of the hand (thumb or radial side), the
first bone in the proximal row is the =scapholunar= (_a_) (equal to the
scaphoid or navicular and lunar of the human hand). It articulates with
the radius. The next is the =cuneiform= (_b_), articulating with the
styloid process of the ulna, and the next, which is attached to the
cuneiform and projects freely ventrad, is the =pisiform= (_c_).
In the distal row the bone on the radial side is the =trapezium= (_d_);
the next is the trapezoid (_e_), the next the os magnum (_f_), and the
last the unciform (_g_). The distal row articulates with the metacarpals
or bones of the palm of the hand (1-5).
In the kitten the scapholunar is represented by three bones, the
=scaphoid= or navicular, on the radial side, the =lunare=, between the
scaphoid and the cuneiform, and a =centrale=, which lies distad of the
other two.
_Scapholunar Bone._ _Os scapholunaris_ (Fig. 51, _a_).--The scapholunar
is a quadrangular bone with the ventroradial angle produced into a blunt
process. Its proximal surface is smooth and articulates with the distal
end of the radius. The distal end is marked by oblique ridges and
articulates with the unciform, os magnum, trapezoid, and trapezium. The
ulnar surface articulates with the cuneiform, and the dorsal surface of
the ventroradial process with the radial sesamoid.
_Cuneiform Bone._ (_Os triquetrum BNA_) (Fig. 51, _b_).--The cuneiform
bone has the form of a flattened pyramid. Its base articulates with the
unciform, its proximoulnar surface with the pisiform except at its
dorsal margin, where it articulates with the styloid process of the
ulna. On its proximoradial surface is a smooth facet for articulation
with the scapholunar.
_Pisiform Bone._ _Os pisiforme_ (Fig. 51, _c_).--The pisiform bone is
about twice as long as broad, with enlarged ends. Its dorsal end
articulates with the cuneiform, and on its proximal surface, separated
from the above by a smooth ridge, is a smooth facet for articulation
with the styloid process of the ulna.
[Illustration: FIG. 51.--CARPUS, METACARPUS, AND PHALANGES, DORSAL
SURFACE.
_a_, scapholunar bone; _b_, cuneiform; _c_, pisiform; _d_, trapezium;
_e_, trapezoid; _f_, os magnum; _g_, unciform; _h_, radial sesamoid;
_i_, proximal phalanges; _j_, second phalanges; _k_, distal
phalanges; 1, 2, 3, 4, 5, metacarpals in order from the radial side.]
_Unciform Bone._ (_Os hamatum BNA_) (Fig. 51, _g_).--The unciform is a
wedge-shaped bone with the apex of the wedge directed proximad, and
smooth for articulation with the scapholunar. By a part of its ulnar
surface it articulates with the cuneiform, and by its radial surface
with the os magnum. Its distal end articulates with the fourth and fifth
metacarpals.
_Os magnum._ (_Os capitatum BNA_) (Fig. 51, _f_).--The os magnum may be
described as an oblong plate bearing on its proximal surface a
semicircular ridge which crosses it diagonally. The proximal end of the
bone articulates with the scapholunar. Its distal end articulates with
the third metacarpal except near its ventroulnar angle, where it
articulates with the fourth metacarpal. Its ulnar surface articulates
with the unciform. Its radial border articulates with the trapezoid, the
third metacarpal, and, by two facets, with the second metacarpal.
_Trapezoid._ (_Os multangulum minus BNA._) (Fig. 51, _e_).--The
trapezoid is somewhat wedge-shaped, with the apex of the wedge pointing
ventrad. Its proximal side articulates with the scapholunar, its distal
side with the second metacarpal, its ulnar side with the os magnum, and
its radial side with the trapezium.
_Trapezium._ (_Os multangulum majus BNA._) (Fig. 51, _d_).--The
trapezium has the form of a triangular prism curved into a semicircle.
The convex face looks proximad and articulates by its ventral half with
the scapholunar. The ulnar surface articulates with the second
metacarpal dorsally, and ventrally with the trapezoid. Its radial
surface articulates with the first metacarpal.
=Bones of the Hand or Manus= (Fig. 51, 1-5).--The _Metacarpals_.
_Metacarpus._--The metacarpals are the five bones of the palm of the
hand; they are numbered from one to five, beginning with the thumb. They
are cylindrical elongated bones with enlarged ends. The distal end is
called the head, and the proximal end the base. Each head bears a
hemispherical articular facet which is marked over its ventral half by a
prominent smooth ridge. The surface dorsad of the ridge articulates with
the proximal head of a phalanx. The ridge and the surface at its sides
are for a pair of sesamoid bones.
The first metacarpal (1) is the shortest. Its head is oblique, and it
articulates by the ulnar half of its proximal surface with the trapezium
(_d_); by the radial half with the radial sesamoid (_h_).
The second metacarpal (2) is marked on the proximal part of its dorsal
surface by an oblique groove passing from the radial side distad to the
ulnar side. The base articulates with the trapezoid (_e_). The ulnar
surface of the proximal end articulates with the os magnum (_f_) and
third metacarpal, while the radial surface articulates with the
trapezium (_d_).
The third metacarpal (3) is the longest, and its base is rhomboidal with
a projecting dorsoradial angle separated by a groove from the rhomboid
surface. The proximal end articulates with the os magnum (_f_) and
second metacarpal; the radial surface of the proximal end with the
second, and the ulnar surface with the fourth, metacarpal.
The fourth metacarpal (4) has a base similar in form to that of the
third, and when placed in position with the fifth the two form a
hemispherical facet which articulates with the unciform (_g_) and os
magnum (_f_). The fourth metacarpal articulates by its radial side with
the third, and by its ulnar side with the fifth.
The fifth metacarpal (5) articulates by its proximal end with the
unciform (_g_), and by the radial side of its proximal end with the
fourth metacarpal.
_Digits_ (Fig. 51).--The first digit of the hand is called the =pollex=
(thumb), the second the =index=, the third the =medius=, the fourth the
=annularis=, the fifth the =minimus=.
The first digit has two phalanges, each of the others three phalanges.
_Phalanges_ (Fig. 51, _i_, _j_, _k_).--The phalanges of the proximal row
(_i_) are elongated, flattened dorsoventrally and curved so as to be
longitudinally convex dorsad. All have thickened ends. The proximal end
is notched, and its proximal surface looks dorsad and is concave for the
head of the metacarpal. The distal end is pulley-shaped, and the pulley
surface extends farther on to the ventral than on to the dorsal surface,
and serves for articulation with the middle phalanx.
The phalanges of the middle row (_j_) are like those of the proximal
row, but shorter. The proximal surface is triangular and marked by a
median facetted ridge. The whole surface is smooth and adapted to the
distal end of the phalanx of the first row. The distal end is
transversely elongated, so as to be cylindrical, and projects more
toward the ulnar than toward the radial side. The distal phalanx (_k_)
articulates with this cylinder so that when it is fully extended it lies
on the ulnar side of the middle phalanx.
The distal phalanx (_k_) has the form of a quadrangular prism. It is
excavated on its proximal surface for articulation with the middle
phalanx. Its distal surface presents dorsad a deep excavation from the
bottom of which arises a compressed plate of bone having the form of a
bird’s beak. The depression receives the base of a claw, and the
beak-like projection supports the claw.
_Sesamoid Bones of the Hand._ _Ossa sesamoidea._--The hand contains, in
addition to those already described, eleven small bones that are
developed in tendons.
One of these, the =radial= sesamoid (Fig. 51, _h_), is closely applied
to the radial end of the scapholunar bone. It is developed in the
tendon of the extensor brevis pollicis muscle.
The other ten occur in pairs as small flattened curved bones on the
ventral side of the joint between each metacarpal and the phalanx with
which it articulates.