MUSCLES OF THE FOREARM AND HAND (_continued_).
3rd. The posterior deep muscles of the forearm; their tendons
at the level of the wrist (_anatomical snuff-box_).--Muscles of
the hand;--1st. Muscles of the thumb, or _thenar eminence_. 2nd.
Muscles of the little finger, or _hypothenar eminence_. 3. Short
muscles of the palm (_lumbricales_ and _interossei_).
III. _The deep posterior muscles_ of the forearm, with regard to
external form, are important only in the arrangement of their tendons
in the wrist and hand; for this reason we describe them in this chapter
along with the muscular structures of the hand and fingers.
As in the other regions, we find in the deep part of the back of the
forearm, four muscles. Proceeding from without inwards they are--the
_extensor ossis metacarpi pollicis_, the _extensor brevis pollicis_,
the _extensor longus pollicis_, and the _extensor indicis_. The fleshy
bellies of these muscles are almost completely hidden beneath the
posterior superficial muscles; but their tendons, at least those of the
first three, emerge beneath the external border of the common extensor
of the fingers, and their form is shown (7 and 8, Fig. 78) by details
of great importance on the outer side of the dorsal aspect of the
wrist.
The two first (7 and 8, Fig. 78), the _extensor ossis metacarpi
pollicis_ (7) and the _extensor brevis pollicis_ (8), must be described
together, as their fleshy parts and tendons are placed together and
are almost united throughout the greater extent of their course. These
two muscles emerge in the lower third of the forearm, at the outer
border of the common extensor of the fingers; and they form at this
point, at the junction of the posterior surface with the outer border
of the forearm, an oblong eminence, which is soon succeeded by a double
tendon. Crossing the tendons of the radial extensors of the wrist, this
double tendon is directed towards the outer surface of the lower end
of the radius, where it passes through a groove (10) converted into
a canal by the posterior annular ligament of the wrist. At the outer
border of the wrist these two tendons form a prominence, well marked
beneath the skin, when the thumb is separated from the other fingers.
Finally, at the base of the metacarpal bone of the thumb these tendons
separate, one, that of the _extensor ossis metacarpi pollicis_, being
inserted into the base of the metacarpal bone, while the other, that of
the _extensor brevis pollicis_, passes over the first metacarpal bone
to be inserted into the base of the first phalanx of the thumb (8, at
the level of the thumb, Fig. 78).
The _extensor longus pollicis_ (9, Fig. 78) emerges, like the preceding
tendons, on the outer side of the common extensor, but lower down, on
the back of the wrist; its tendon only becomes superficial at this
point, and it is directed obliquely downwards through a prominent
deep groove on the posterior surface of the lower end of the radius,
where it is covered by the posterior annular ligament. On the back
of the wrist the tendon is directed obliquely outwards, crossing the
tendons of the radial extensors of the wrist, to reach the base of the
metacarpal bone of the thumb, where it lies parallel to the tendon of
the extensor brevis pollicis. Descending lower than this tendon it is
finally inserted into the second or terminal phalanx of the thumb (9,
Fig. 78).
The tendons of the extensor ossis metacarpi and extensor brevis
pollicis on the one hand, and the extensor longus pollicis on the
other, form on the outer part of the back of the wrist a triangular
figure, of which the apex corresponds to the upper end of the thumb,
and the base to the lower end of the radius. When we separate the thumb
strongly from the index finger--that is to say, when we contract the
three small muscles which we have been studying--the corresponding
tendons mark the borders of this triangle in the form of prominent
cords, between which is a deep concavity; to this depression is given
the name of the _anatomical snuff-box_ (4, Fig. 78).
The _extensor indicis_ is not visible on the model; it is deeply
situated beneath the common extensor of the fingers, and terminates
in a tendon (13, Fig. 78) which unites with the tendinous fibres from
the common extensor to the index finger. It is to this muscle that
the index finger owes its power of extension independent of the other
fingers, and of performing the functions which have given it the name
of the index or indicating finger. It is to be noted that in separate
extension of the index finger (with flexion of the others) the tendon
of the extensor communis digitorum in the back of the hand gets curved
outwards, by being pulled into line with the extended index finger by
the traction of the tendon of the extensor indicis (Macewen).
[Illustration: FIG. 79.
MUSCLES OF THE HAND (palmar surface).--2, abductor pollicis
(cut);--3, opponens pollicis;--4, flexor brevis pollicis;--5,
adductor obliquus pollicis;--6, adductor transversus
pollicis;--7, deep flexor tendons and lumbrical muscles;--8,
flexor tendons on finger;--9, flexor longus pollicis;--10,
abductor minimi digiti;--11, flexor brevis minimi digiti;--12,
pisiform bone.]
_Muscles of the hand._--The numerous muscles belonging to the hand
form an interesting study in relation to the mechanism of the multiple
and delicate movements of the fingers, but as the various details of
their complex arrangement do not show very plainly on the surface we
may confine our study of them to an enumeration of their principal
features.
The dorsal region of the hand (Fig. 78) does not possess any fleshy
muscles, but only presents the tendons belonging to the muscles of
the forearm. The arrangement of the tendons after passing beneath the
posterior annular ligament has already been described (pp. 245–6).
Notice should be taken of an arch of veins lying superficial to the
tendons. This venous arch receives the veins from the fingers, and is
variable in its arrangement and outline.
On the other hand, the anterior or palmar region of the hand possesses,
beside the tendons of the muscles of the forearm, numerous small
muscles, which are divided into three groups:--1st, an external group
associated with the thumb, forming the fleshy prominence known as the
_thenar eminence_; 2nd, an internal group, belonging to the little
finger, forming the _hypothenar eminence_; 3rd, a middle and deeper
group, formed by small muscles belonging to the fingers, and placed
deep in the hand beneath the thick _palmar fascia_, which, occupying
the hollow of the palm, fills up the space between the thenar and
hypothenar eminences (Fig. 79).
1st. The _thenar eminence_ (2–5, Fig. 79) is of a long ovoid form,
with the large superior extremity corresponding to the carpus, and
the smaller inferior extremity corresponding to the base of the
first phalanx of the thumb. It is formed by six muscles, namely, the
_abductor pollicis_ (24, Fig. 77; 2, Fig. 79), which proceeds from
the trapezium and annular ligament to the outer side of the first
phalanx of the thumb; the _opponens pollicis_ (3, Fig. 79), with a
similar origin, inserted into the entire length of the outer border
of the first metacarpal bone, so that its contraction draws the whole
thumb (phalanges and metacarpal bone) towards the palm of the hand,
and thus opposes it to the other fingers; the _flexor brevis pollicis_
(superficial part) (4, Fig. 79), which proceeds from the annular
ligament to the base of the first phalanx; the _adductor obliquus
pollicis_ (5, Fig. 79), arising from the carpus and heads of the
metacarpal bones, and inserted into the inner side of the first phalanx
of the thumb; the _adductor transversus pollicis_ (6, Fig. 79), a
muscle remarkable for its arrangement, for it arises from the middle of
the palm of the hand, from the anterior surface of the third metacarpal
bone, and forming a comparatively broad fleshy mass, extends outwards
to be inserted along with the adductor obliquus into the inner side of
the base of the first phalanx of the thumb; and finally a small muscle
deeply placed in the space between the first and second metacarpal
bones, and known as the _deep part of the flexor brevis pollicis_ (or
the _interosseus primus volaris_).
2nd. The _hypothenar eminence_ is long and elliptical in form, but
smaller than the thenar eminence; it is covered in part by a small
muscle which is not shown by any external prominence, but only by
the folds which it marks in the skin during its contraction; this is
the _palmaris brevis_, formed of transverse fibres, which, arising
from the inner border of the palmar fascia, is inserted into the deep
surface of the skin on the inner border of the hand. The contraction of
its fibres draws the skin of the part upwards and forms an irregular
vertical furrow, so that the prominence of the skin of the upper part
of the hypothenar eminence becomes more clearly marked. The hypothenar
eminence itself is formed by three small muscles vertically arranged
over the fifth metacarpal bone:--1, the _abductor minimi digiti_ (28,
Fig. 77; 10, Fig. 79), passing from the pisiform bone to the inner side
of the first phalanx of the little finger; 2, the _flexor brevis minimi
digiti_ (27, Fig. 77; 11, Fig. 79), passing from the prominence of the
unciform bone to the same phalanx; and lastly, 3, the _opponens minimi
digiti_, arising from the unciform bone and inserted into the entire
length of the fifth metacarpal bone, so that its contraction slightly
draws outwards the whole of the little finger, and opposes it to a
certain degree to the thumb.
3rd. The muscles of the middle region of the palm of the hand are in
two series. One set is arranged between the tendons of the _flexor
profundus digitorum_ muscle (see anterior deep muscles of the forearm,
page 237) as small fusiform muscles, which have been compared to the
form of a worm--hence their name of _lumbricales_ (7, Fig. 79). The
others are arranged in the spaces between the metacarpal bones, and are
known as the _interosseous_ muscles.
The _lumbricales_ muscles, as Figure 77 shows (23, 23), are four in
number--one for each of the four fingers. Their upper ends are attached
to the deep flexor tendons; and from this origin they descend to
reach the outer or radial border of the first phalanx of each finger.
Sweeping round the knuckle on its outer side, each tendon is inserted
into the metacarpo-phalangeal capsule, the outer side of the base of
the first phalanx, and into the expansion of the extensor tendon on the
dorsum of the first phalanx of each finger. In consequence of this mode
of insertion each muscle is a flexor of the metacarpo-phalangeal joint,
and in addition, because of its connection with the extensor tendon, it
is at the same time an extensor of the inter-phalangeal joints.
The _interosseous muscles_ are seven in number, divided into two
series, dorsal and palmar. The _dorsal_ muscles, four in number, are
found one in each interosseous space between the metacarpal bones. The
three _palmar_ interosseous muscles occupy the three inner spaces.
They arise from the sides of the metacarpal bones (the dorsal muscles
by double origins, the palmar muscles by single heads from the inner
three metacarpal bones); and they are inserted in the same way as the
lumbrical muscles into the fingers, so as to act as flexors of the
metacarpo-phalangeal, and extensors of the inter-phalangeal joints. The
several muscles pass in relation to the sides of the knuckles in a way
that need not be detailed here, so as to occupy positions which enable
them to act so as to separate or draw together the fingers. The dorsal
interossei muscles serve to separate the fingers from each other, while
the palmar serve to bring them together; or, to be more precise, the
dorsal muscles abduct the fingers from the middle line of the middle
finger, while the palmar muscles adduct the fingers on which they act
(third, fourth, and fifth) towards the axis of the middle finger.