MUSCLES OF THE ARM.
1st. Anterior muscles: _Biceps_, its two heads; its fusiform
body; its bifurcation below (aponeurotic expansion and
tendon); its action (supinator and flexor of forearm);
influence of its aponeurotic expansion on the contour of the
forearm.--_Coraco-brachialis_, _brachialis anticus_. 2nd.
Posterior muscle: _Triceps brachialis_, its three portions, flat
surface formed by its inferior tendon; general contour of the
posterior surface of the arm; action of triceps. External form
of the arm; external and internal intermuscular grooves.
The muscles of the arm form two distinct fleshy masses, a mass in front
formed by the _biceps_, which occupies the entire length of the arm;
by the _coraco-brachialis_, which occupies only the upper part; and
lastly, by the _brachialis anticus_, which occupies the lower part; and
a mass behind, formed by one muscle only, the _triceps_.
The _biceps_ (12, Fig. 75, and 21, Fig. 70) is so called because it
possesses two heads of origin, which are known by the names of the
_long_ and _short heads_. The _long head_ of the muscle presents the
form of a long tendon, which arises from the upper limit of the glenoid
cavity of the scapula, and descends through the shoulder-joint and the
bicipital groove of the humerus to the arm. The _short head_ of the
biceps has a less complex course, and arises from the summit of the
coracoid process, in common with the coraco-brachialis.
[Illustration: FIG. 75.
THE MUSCLES OF THE SHOULDER AND ARM (seen from the external
side).--1, the triceps brachialis;--2, its long head;--3, its
external head;--4, its attachment to the olecranon;--5, the
teres major;--5′, the teres minor;--6, the infraspinatus;--7,
8, 9, 10, the deltoid;--11, the supraspinatus;--12, the biceps
brachialis;--13, the brachialis anticus;--14, the supinator
longus;--15, extensor carpi radialis longior.]
These two tendons (long and short head) descend in the outer wall of
the axilla, covered by the great pectoral muscle (Fig. 70, page 197);
a little above the lower border of this muscle the fleshy fibres appear
and form two cylindrical bellies which descend and become united, at
the level of the middle of the anterior surface of the arm, in one
large mass, very marked in muscular subjects (12, Fig. 75). A short
distance above the elbow-joint, a flat tendon of insertion succeeds
to the fleshy muscle; it is at first broad, but soon divides into two
parts, one aponeurotic, called the _semi-lunar fascia_ of the biceps;
the other, a rounded tendon forming the chief insertion of the muscle.
The aponeurotic expansion (2, Fig. 77) of the bicipital fascia is
directed downwards and inwards over the common mass of the flexor
muscles of the forearm, and soon becomes blended with the aponeurosis
of the forearm; the tendon proper (3, Fig. 74) dips down in the hollow
of the elbow between the internal and external muscles of the forearm,
and is fixed to the bicipital tubercle of the radius (page 78, and Fig.
21), upon which it turns to be inserted into the posterior part of the
tubercle.
The biceps muscle is essentially the flexor of the forearm on the arm.
This action is evident, and known to all, and it is useless to dwell
on it, except to recall the fact that the biceps, in acting on the
forearm, is inserted perpendicularly on the lever which it moves, so
that it is in the most favourable position for the exercise of all its
power. But the contraction of the biceps produces along with flexion
of the elbow two other movements to which it is important to direct
attention. 1st. If the forearm is in pronation the tendon of the
biceps is obviously twisted round the upper part of the radius, since
it is inserted into the posterior part of the bicipital tubercle; the
first effect produced, therefore, by its contraction is a rotation
of the radius outwards, and a movement of supination; the biceps is,
therefore, a supinator, and one of the most powerful. 2nd. Besides
flexion at the elbow-joint and this movement of supination, the biceps
muscle raises and draws inwards the humerus by its action on the
shoulder-joint. The three separate movements are found in association
with one another in such a movement as the act of raising the hand to
the mouth.
During the contraction of the biceps, its aponeurotic expansion is
tightened, and binds down the mass of the flexor muscles of the forearm
over which it passes; hence it marks on the inner surface of the
fleshy part of this portion of the limb, two fingers’ breadth below the
internal condyloid ridge, a distinct furrow, during the contraction of
the muscle.
The changes of form in the arm which accompany the contraction of the
biceps are well known, and it is sufficient to recall the fact that
the fleshy belly of this fusiform muscle is lengthened in the state of
repose, becoming short and globular during contraction. Nothing is more
striking, nor gives a better idea of the change of form in a muscle
during contraction, than to examine the biceps in a model, who brings
it gradually into action by flexing the forearm on the arm (Fig. 76).
We then see on the anterior surface of the arm a kind of fleshy ball
becoming more and more clearly marked, which hardens and swells up at
the same time, so that it seems to mount up towards the upper part of
the arm, to the inferior border of the great pectoral muscle.
The _coraco-brachialis_ muscle forms a narrow fusiform fleshy mass,
which occupies the upper part of the inner surface of the arm. It
arises from the coracoid process of the scapula (alongside the short
head of the biceps), and is inserted into the middle of the internal
border of the humerus. When the arm is hanging loosely the lower half
only of this muscle is visible on the subject, and its prominence is
blended with that of the biceps, while its upper half is hidden within
the axillary space, beneath the great pectoral muscle. It becomes
visible beneath the skin of the armpit, however, when the arms are
raised, as in the position on the cross, and we have already dwelt
upon the contour which the fusiform muscular belly of this muscle
presents in the outer wall of the axilla. When the coraco-brachialis
contracts, its form becomes more prominent, like that of every muscle
in contraction; but it is not more visible on this account, for as it
draws the arm to the thorax against which it is applied, it conceals by
this movement the region in which its prominence is marked.
[Illustration: FIG. 76.
A, the arm extended; B, the elbow flexed and the biceps
contracted.]
The _brachialis anticus_ muscle (13, Fig. 75, and 4, 4, Fig.
77).--Situated beneath the lower half of the biceps, which it overlaps
on each side, this muscle is deeply placed, and covers the lower
two-thirds of the anterior surface of the humerus, to which it is
attached. The fleshy fibres descend almost to the level of the elbow,
where they are replaced by a flat tendon, inserted into the front of
the coronoid process of the ulna. As the ulna does not exhibit any
of the lateral movements comprised in supination and pronation, the
brachialis anticus is simply a flexor of the forearm, and when this
movement is forcibly accomplished, it may be seen to swell up on each
side of the lower part of the biceps.
_Triceps muscle._--This muscle (21, 22, Fig. 73; 1, 2, 3, Fig. 75),
which forms alone the entire muscular structure of the posterior
surface of the arm, has been called the _triceps_ because it arises
by three heads, separated above, united below; one central or median,
called the _long head_, and two lateral parts, distinguished as the
_external_ and _internal heads_. The _long head_ (2, Fig. 75), forming
a thick and fusiform fleshy belly, arises by a stout tendon from the
upper part of the axillary border of the scapula immediately below the
glenoid cavity, and passes between the teres major and teres minor
muscles to the back of the arm (page 223). At the level of the junction
of the middle with the lower third of the arm, the fleshy belly
terminates in a broad, flat tendon (1, Figs. 75 and 78) which forms the
main insertion of the muscle, and receives the insertions of the two
other portions of the muscle. The _external head_ (3, Fig. 75) arises
from the upper part of the posterior surface of the humerus (above and
to the outer side of the spiral groove), and is directed obliquely
downwards and inwards to be attached to the external border of the main
tendon almost as low down as the elbow. Lastly, the _internal head_
(22, Fig. 73) arises from the lower part of the posterior surface of
the humerus (below and to the inner side of the spiral groove), and is
attached to the internal border and deep surface of the inferior common
tendon. This tendon is inserted (4, Fig. 75) into the upper surface of
the olecranon process of the ulna, and on either side of this it blends
with the deep fascia of the forearm.
The form of the posterior surface of the arm is greatly influenced
by the presence of the inferior common tendon of the triceps, which,
narrow and pointed towards its upper end, forms a large flat surface
at the lower part of the arm. This flat surface is overlapped on each
side by the prominence of the fleshy parts of the inner and outer
heads. Above, in the upper two-thirds of the posterior surface of the
arm, two fleshy bellies may be seen side by side, an outer one formed
by the outer head, an inner one formed by the long head of the muscle.
The inner head, more deeply placed, is only obvious in the lower third
of the arm. These various details--viz., the tendinous flat surface
above the olecranon, the muscular prominences which bound it on each
side, and the two fleshy bellies which ascend above it--become visible
in a marked degree when the subject forcibly extends the forearm on the
arm, as in struggling against any resistance which causes the position
of flexion to be maintained. It is hardly necessary to say that the
triceps is essentially the extensor muscle of the forearm on the arm.
It is not only for the purpose of a regular enumeration, but also
with regard to the anatomical interpretation of their external forms,
that we have classed the muscles of the arm as anterior and posterior.
On each side of the arm at both inner and outer borders, a groove
runs which separates the anterior from the posterior muscles. In
each of these grooves is a fibrous partition called the internal and
external intermuscular septum, which extends from the surface to the
corresponding supra-condyloid ridge of the humerus, and serves to
increase the area of muscular attachment. In muscular contraction,
therefore, each septum or aponeurosis is slightly drawn towards
the humerus, and on the surface two grooves are clearly seen, each
corresponding to an interval, internally or externally, between the
anterior and posterior muscles.
The internal groove commences at the inferior extremity of the
coraco-brachialis, and descends almost to the inner condyle. Above it
is faintly marked, because the numerous nerves and vessels surrounded
by cellular tissue fill up the intermuscular space at this level;
below it spreads out, and tends to be confounded with the form of the
internal part of the brachialis anticus.
The external groove (Fig. 75) is short. It begins at the level of the
lower extremity of the deltoid, and does not descend to the external
condyle, because the highest muscles of the extensor group belonging
to the forearm arise from the lower part of the outer border of the
humerus, so that the groove is filled up by these muscles as they curve
forwards towards the bend of the elbow.