MUSCLES OF THE FOREARM AND HAND.
Division into groups:--A. Anterior muscles of the forearm.
The _superficial muscles_, or _those arising from the inner
condyle_: pronator radii teres, flexor carpi radialis, palmaris
longus, and flexor carpi ulnaris. 2nd. The _deep muscles_
(flexors of the fingers and pronator quadratus). B. Posterior
muscles of the forearm. 1st. _External muscles_: supinator
longus (importance with regard to the forms of the external
region of elbow); the extensor carpi radialis, longior, and
brevior; the supinator radii brevis. 2nd. The _posterior
superficial muscles_: the extensor communis digitorum, extensor
minimi digiti, extensor carpi ulnaris, and anconeus.
_Muscles of the Forearm._--The two bones of the forearm are covered
by a series of muscles with fleshy bellies, generally fusiform,
terminating inferiorly in tendons which are frequently very long, and
become prominent in the region of the wrist. Some of these muscles move
the forearm on the arm, or the radius on the ulna, but the action of
the greater number is to move the hand on the forearm and the different
segments of the fingers on each other. These muscles are divided into
five series, each composed of four muscles, making the total number of
muscles in the arm twenty altogether. We shall dwell chiefly upon the
superficial muscles, a short mention being sufficient for the deeper
ones. We distinguish first of all two groups, anterior and posterior:
one on the front, the other on the back of the forearm. The anterior
group is divisible into: 1st. An anterior superficial layer, of which
each muscle will be studied; 2nd. An anterior deep layer, to which we
will briefly refer. The posterior group may be subdivided into three:
1st. An external layer; 2nd. A superficial posterior layer, which we
must review in detail; and 3rd. A posterior deep layer, respecting
which we shall only say sufficient to enable the reader to understand
the shape of the wrist formed by the corresponding tendons.
A. _ANTERIOR MUSCLES OF THE FOREARM._ I. _Anterior superficial
muscles._--All these muscles arise by a common tendon from the
_internal condyle_ of the humerus; their common origin does not
appreciably extend above the level of the condyle, so that at the inner
side of the elbow, contrary to what takes place on the outer side,
the muscles of the forearm do not ascend on the corresponding side of
the upper arm. If from the inner condyle we draw four lines, of which
the first goes towards the middle of the radius, the second towards
the outer border of the wrist, the third towards the middle, and the
fourth towards the inner border of the wrist; these four lines, of
which the first is very oblique and the others gradually approach the
vertical, will give us the direction of each of these four anterior
superficial muscles of the forearm, which are, in the order of the
lines, from without inwards, the _pronator radii teres_, the _flexor
carpi radialis_, the _palmaris longus_, and the _flexor carpi ulnaris_.
[Illustration:
FIG. 77.--THE ANTERIOR MUSCLES OF THE LEFT FOREARM.--1,
the biceps brachialis;--2, its aponeurotic expansion;--3,
its tendon;--4, 4, brachialis anticus;--5, the internal
head of the triceps;--6, pronator radii teres;--7, flexor
carpi radialis;--8, 9, palmaris longus;--10, flexor carpi
ulnaris;--11, its attachment to the pisiform bone;--12, 13,
supinator longus;--14 and 15, the extensor carpi radialis,
longior, and brevior;--16, the abductor longus pollicis;--17,
its tendon;--18, tendon of extensor longus pollicis;--19, 20,
21, the superficial flexor of the fingers and its tendons;--22,
tendons of the deep flexors;--23, 23, the lumbricales;--24,
abductor brevis pollicis;--26, flexor longus pollicis.]
The _pronator teres_ (6, Fig. 77) is fleshy throughout the entire
extent in which it is visible on the model; arising from the internal
condyle, it is directed obliquely downwards and outwards, and
disappears beneath the external muscles of the forearm (supinator
longus) to be inserted into the radius (around which it is slightly
twisted) in the middle of its external surface (impression for the
pronator teres, page 78). Contracting, the pronator teres turns the
radius forward and inwards, producing pronation. This muscle forms the
inner side, very oblique, of a triangular pit, known as the _hollow
of the elbow_, of which the outer side is formed by the supinator
longus (12, Fig. 77). In this hollow the tendon of the biceps dips down
(3, Fig. 77) along with the brachialis anticus (4, 4, Fig. 77) to be
inserted into the bones of the forearm. The upper part of the pronator
teres is crossed by the aponeurotic expansion of the biceps (2, Fig.
77), and we have already dwelt on the particulars of external form
which result from this arrangement.
The _flexor carpi radialis_ (7, Fig. 77) arises from the inner condyle
by a fusiform fleshy belly which descends obliquely to the middle of
the forearm. It is replaced by a narrow, strong tendon which gains
the outer part of the wrist, and then disappears beneath the annular
ligament of the carpus, and, lying in a groove on the anterior surface
of the trapezium, is inserted into the bases of the metacarpal bones
of the fore and middle fingers. This muscle flexes the hand on the
forearm; when it contracts, its tendon becomes very prominent and
raises the skin at the lower part of the front of the forearm; it forms
the first tendinous prominence that we meet at this level in passing
from the radial to the ulnar border, and lies just internal to the
groove in which the radial artery pulsates.
The _palmaris longus_ (8, 9, Fig. 77) is much smaller than the
preceding muscle; arising from the internal condyle, it forms a short
fusiform fleshy belly (8) to which a long slender tendon succeeds.
Descending almost vertically to the middle of the wrist, the tendon
ends by being inserted into the annular ligament of the carpus and the
fascia of the palm (9, Fig. 77). The palmaris longus flexes the hand
on the forearm, and its tendon forms in contraction of the muscle a
well-marked prominence situated in the middle of the lower part of the
front of the forearm on the inner side of the tendon of the flexor
carpi radialis. In some subjects this muscle is absent, and it is
frequently subject to variations of form.
The _flexor carpi ulnaris_ (10, 11, Fig. 77) arises not only from the
inner condyle, like the three preceding muscles, but also (18, Fig.
78) from the posterior border of the ulna (including the olecranon)
process; it descends vertically over the ulna, and is inserted by
tendinous and fleshy fibres into the pisiform bone of the carpus (11,
Fig. 77). As its fleshy fibres continue down to its insertion the form
of the muscle is not marked by a prominence such as those produced by
the tendons of the preceding muscles; this muscle, therefore, helps
to give a rounded form to the whole extent of the inner border of the
forearm. It flexes the hand, and at the same time draws it inwards
towards the inner side of the wrist.
II. _Anterior deep muscles._--These muscles constitute a fleshy mass
lying beneath the preceding superficial muscles, and terminate near
the wrist in numerous tendons, which pass to the fingers; the tendons
of the muscles appear to a slight extent on the model in the grooves
between the tendons of the superficial muscles (19, 19, Fig. 77). Lower
down the tendons of these muscles lie in the anterior groove of the
carpus, bound down by the corresponding annular ligament, and after
traversing the hollow of the palm of the hand, they are inserted into
the phalanges of the fingers, presenting characteristic features which
we will rapidly point out in the enumeration of these muscles.
These deep muscles comprise, 1st, the _flexor sublimis digitorum_
(19, Fig. 77), which divides below into four tendons, one for each
finger (except the thumb); 2nd, the _flexor profundus digitorum_,
which also divides into four tendons, one for each finger (except the
thumb). On the anterior surface of each finger, therefore, we see two
tendons--one superficial (23, Fig. 77) and the other deep. The first
(flexor sublimis) presents at the level of the first phalanx (20, Fig.
77) a slit or button-hole, through which the second passes (flexor
profundus); owing to this arrangement the tendon of the deep flexor is
inserted into the base of the third phalanx (22, Fig. 77); while the
superficial flexor is inserted into the base of the second phalanx (21,
Fig. 77). There is, indeed, a flexor muscle for each of the phalanges;
the first phalanges of the fingers having special flexors in addition,
the small _lumbricales_ muscles of the palm of the hand, which arise
from the sides of the tendons of the flexor profundus digitorum in the
palm of the hand.
3rd. The _flexor longus pollicis_ (26, Fig. 77) arises in the forearm
from the radius, and its tendon is inserted into the base of the second
or last phalanx of the thumb. It gives no evidence of its existence
on the surface. 4th. The _pronator quadratus_ muscle, a quadrilateral
fleshy muscle, is disposed in a different manner to the preceding
muscles, which must be raised in order to see it. It is formed by
transverse fibres placed in the lower fourth of the forearm, and it
passes from the anterior surface of the ulna to that of the radius. Its
contraction produces pronation, rotating the radius over the lower part
of the shaft of the ulna.
B. _POSTERIOR MUSCLES OF THE FOREARM._ I. _External muscles._--These
form the fleshy mass which covers the lower third of the outer border
of the arm (Fig. 75, page 225) and descends along the outer border
of the radius in the forearm. The group comprises four muscles--the
_supinator longus_ (_brachio-radialis_), two _radial extensors of the
carpus_, and the _supinator radii brevis_.
Of these four muscles, one only is visible on the model throughout its
entire extent--viz., 1st, the _supinator longus_ or _brachio-radialis_
(14, Fig. 75; 12, 13, Fig. 77), which arises from the outer
supra-condyloid border of the humerus, between the brachialis anticus
in front and the triceps behind. It is noteworthy that on this side the
muscles of the forearm ascend on the arm almost up to the insertion
of the deltoid. The supinator longus enlarges as it descends, so that
it presents its greatest size at the level of the external condyle,
the prominence of which it completely conceals. It forms the outer
vertical boundary (page 234) of the triangular hollow at the bend of
the elbow. Just below the middle of the forearm, the fleshy fibres
are replaced by a long tendon which lies on the radius, and (13, Fig.
77) is finally inserted into the base of the styloid process of that
bone. Notwithstanding its name, this muscle is not essentially a
supinator; it acts in this way only when the forearm is in a position
of complete pronation, and its proper action in this respect is to
bring the forearm into a position between supination and pronation.
Its principal function, however, is the flexion of the forearm on the
arm, and in this movement its form is clearly shown externally in the
form of a prominent band, which rises from the arm, and forms, on the
antero-external part of the elbow, a strong fleshy mass, filling up the
hollow produced by the flexion of the forearm on the arm. The supinator
longus is the most important of the muscles of the forearm with regard
to the part it takes in the external form of this region.
The two next muscles (14 and 15, Fig. 77) are partly hidden by the
preceding; they are the _radial extensors of the carpus_ (2 and
3), distinguished as the _extensor carpi radialis_, _longior_ and
_brevior_ (15, Fig. 75). They arise from the lower part of the outer
supra-condyloid ridge of the humerus and from the outer condyle, and
form fleshy masses (3 and 5, Fig. 78), which increase the prominence
of the supinator longus, and help to conceal the outer condyle. At
about the same level as in the case of the supinator longus, a tendon
succeeds the fleshy belly of each of these muscles, and inclines a
little backward (Fig. 78); having been crossed by the _extensor ossis
metacarpi pollicis_, by the _extensor brevis pollicis_ (7 and 8, Fig.
78), and by the _extensor longus pollicis_; each reaches the dorsal
surface of the wrist, and is inserted into the base of the metacarpal
bone of the index finger (extensor carpi radialis longior), and into
the base of the metacarpal bone of the middle finger (extensor carpi
radialis brevior, 6, Fig. 78).
4th. Surrounding the upper part of the radius is a small deep
muscle, which does not show on the model, and which we mention here
only to point out that its presence increases the prominence of the
muscular mass on the outer side of the elbow; this is the _supinator
radii brevis_, formed by fibres which, arising from the humerus,
elbow-joint, and ulna, are rolled round the back and outer side of
the radius, and which acts so as to turn the bone backwards and thus
produce supination.
II. _Posterior superficial muscles_ (Fig. 78).--This group comprises
the _extensor cemmunis digitorum_, the _extensor minimi digiti_, the
_extensor carpi ulnaris_, and the _anconeus_. These four muscles arise
from the external condyle, to which they are attached by a common
tendon; from this origin they are directed downwards, the first almost
vertically, the last (_anconeus_) very obliquely backwards and inwards.
[Illustration:
FIG. 78.--THE POSTERIOR MUSCLES OF THE LEFT FOREARM.--1, the
tendon of triceps brachialis;--2, supinator longus;--3 and
4, extensor carpi radialis longior;--5 and 6, extensor carpi
radialis brevior;--7 and 8, extensor ossis metacarpi pollicis
and extensor brevis pollicis;--9, 9, the extensor longus
pollicis;--10, 10, the annular ligament of the wrist (dorsal
aspect);--11, 12, the extensor communis digitorum and its
tendons;--13, the tendon of the extensor indicis;--14, the
tendon of the extensor minimi digiti;--15, 16, extensor carpi
ulnaris;--17, anconeus;--18, the flexor carpi ulnaris;--19, the
posterior border of the ulna;--20, olecranon;--21, the inner
condyle.]
1st. The _extensor communis digitorum_ (11, Fig. 78) forms a long
fusiform fleshy belly. In the lower third of the posterior surface of
the forearm, it gives place to a tendon which soon subdivides into four
bands. These remain in contact until they have passed a groove situated
in the centre of the inferior extremity of the radius, and separate on
the dorsal surface of the wrist, diverging so as to be attached to each
of the four fingers. On the back of the hand the tendons of the middle,
ring, and little fingers are joined together by two oblique tendinous
slips directed obliquely downwards and outwards, from the fourth to the
third, and from the third to the second tendon respectively. On the
dorsal surface of the first phalanx of each finger (12, Fig. 78) the
extensor tendon forms a membranous expansion, and thereafter divides
into three slips, a central one inserted into the base of the second
phalanx, and two lateral slips which unite again to be inserted into
the base of the third phalanx. The tendons cover the backs of the
knuckles and of the joints of the fingers.
2nd. The _extensor minimi digiti_ is a small bundle of fleshy fibres
lying adjacent to the preceding muscle, but distinctly detached from
it. It gives rise to a separate tendon, which passes through a special
groove beneath the posterior annular ligament between the lower ends
of the radius and ulna. From the wrist it is directed towards the
posterior surface of the little finger (14, Fig. 78), where it unites
with the tendinous fasciculi from the common extensor tendon for the
little finger, and joins in the formation of the membranous expansion
on the dorsum of the first phalanx.
3rd. The _extensor carpi ulnaris_ (15, Fig. 78) is a fusiform muscle
arising from the external condyle and the posterior surface of the
ulna. It is replaced by a tendon in the lower fourth of the forearm
(15, Fig. 78). This tendon passes beneath the posterior annular
ligament in a groove on the posterior surface of the ulna (10, Fig.
78), and at the inner part of the dorsal surface of the carpus it
terminates almost immediately by being inserted into the base of the
metacarpal bone of the little finger (16, Fig. 78).
These three muscles are extensors of the fingers and hand. If we
examine a living model which has the arm folded on the trunk, the
dorsal surface of the forearm being turned forwards, and which quickly
moves the fingers and hand, we see clearly the movements of the fingers
marked by muscular movements in the upper two-thirds of the posterior
surface of the forearm. We can, by following their prominences in
contraction, clearly recognise the fusiform bellies of these muscles.
4th. The _anconeus_ occupies the upper part of the posterior
surface of the forearm. As its name indicates (ἀγκὼν, olecranon,
elbow), it is a muscle of the region of the elbow; it forms (17, Fig.
78) a triangular fleshy mass, of which the apex is attached to the
external condyle, and the base is inserted into the external surface
of the olecranon, and corresponding part of the ulna (19, Fig. 78). As
the ulna does not possess appreciably the movement of rotation, but
shares only in flexion and extension of the elbow-joint, the anconeus
situated behind the joint has no other action than that of extending
the forearm on the arm. When this movement is forcibly produced, we
see the anconeus clearly marked by a triangular prominence, of which
the upper border, the shortest, is united with the prominence of the
triceps; and we have already mentioned the anatomical fact that the
inferior portion of the triceps is directly continuous in the forearm
with the anconeus muscle.