MUSCLES OF THE NECK.
Lateral and anterior regions of the neck.--_Sterno-mastoid_
muscles. The anterior triangle; infra-hyoid and
supra-hyoid regions.--Organs contained in the neck
(vertebral column, œsophagus, and trachea).--Infra-hyoid
muscles--_omo-hyoid_, _sterno-hyoid_, _sterno-thyroid_, and
_thyro-hyoid_.--Supra-hyoid muscles--_digastric_, _stylo-hyoid_,
and _mylo-hyoid_.
_Muscles of the neck._--We have already, in treating of the trapezius,
studied the muscular structure and form of the posterior region of
the neck, and the details have been described of the upper part of
the side of the neck, and the oblique longitudinal groove between
the anterior border of the trapezius and the posterior border of the
sterno-cleido-mastoid (pages 208 and 210). It now remains to examine
the inferior part of this groove and all the anterior region of the
neck. This study should begin with that of the sterno-cleido-mastoid
muscles, which have the most important influence on the form of this
region, and which by their direction map out on the anterior surface of
the neck a muscular interval or space in which it will be easy to study
the deeply placed muscles.
The _sterno-cleido-mastoid muscles_ are two in number, one on each side
of the neck, extending from the upper limit of the thorax, obliquely
upwards and backwards, to the base of the skull (19, Fig. 88). The
muscle arises below by two heads--one internal or sternal, which
arises by a strong tendon from the front of the first piece of the
sternum (18, Fig. 70, page 197), the other external or clavicular,
which arises in the form of a thin fleshy band from the inner third of
the upper surface of the clavicle, above the origin of the clavicular
portion of the great pectoral muscle (Fig. 68, page 194). These
two heads are directed upwards and backwards, separated at first
by a narrow triangular space, of which the base corresponds to the
sterno-clavicular joint (Fig. 68); they then unite to form a single
muscle, broad and thick, which ascends obliquely towards the skull,
passing behind the angle of the lower jaw and the ear to be inserted
into the surface of the mastoid process of the temporal bone and the
superior curved line of the occipital bone (16, Fig. 73).
This muscle, being inserted into the skull at a point which is situated
behind the axis of the movements of flexion and extension of the head,
acts by extending the head on the neck, but to this movement, usually
little marked, it adds the power of flexing the neck on the trunk. When
both these muscles contract at the same time, they produce extension of
the head on the neck and flexion of the neck on the thorax; therefore
we see both clearly marked beneath the skin in a person lying down
who raises his head (by flexion of the neck). When one muscle only
contracts, it turns the face to the opposite side; therefore in the
living model whose face is turned to the right the face turns to this
side by the contraction of the left sterno-cleido-mastoid muscle, of
which the prominence is well shown beneath the skin as a broad band,
passing from the sternum to the mastoid process of the temporal bone.
There are various attitudes in which this prominence is particularly
remarkable, as when we carry the head quickly to one side to answer a
question or give an order; or in the act of listening intently, when
we concentrate our attention to one side and extend the head a little,
turning the region of the ear upwards and forwards, an attitude in
which the sterno-mastoid becomes particularly prominent beneath the
skin of the neck.
From their insertions and direction we see that these two muscles
are very close to each other below and wide apart above. Stretching
diagonally across the side of the neck, each muscle forms the interval
between two triangular spaces--the _posterior triangle_ behind,
bounded by the sterno-mastoid, trapezius, and the clavicle (below);
and the _anterior triangle_ in front, bounded by the sterno-mastoid,
the middle line of the neck in front, and the lower jaw above. The
anterior triangles of the two sides comprise together the space between
the two sterno-cleido-mastoid muscles. This space is in the form of a
triangle, with its apex below, corresponding to the episternal notch,
and its base above, formed by the whole breadth of the lower jaw.
This triangle is the anterior region of the neck, divided into two
parts by the presence of the hyoid bone--a small bone without direct
connection with the rest of the skeleton, and placed transversely above
the prominence of the larynx (23, Fig. 88). The lower part forms a
vertical plane, oblique downwards and backwards, and sinking behind
the sternum; this is the infra-hyoid region, containing the infra-hyoid
muscles. The upper part of the area forms a plane approaching more or
less the horizontal, proceeding from the hyoid bone to the chin and the
circumference of the lower jaw; this is the supra-hyoid region, or that
of the chin, containing the supra-hyoid muscles.
Before entering further into the description of these regions we
must point out the presence of several organs which, placed in front
of the vertebral column, project forwards in the space between the
sterno-mastoid muscles. In the middle line of the neck, the region
above the hyoid bone corresponds to the floor of the mouth, in which
the tongue lies, having on each side large salivary glands partly
concealed by the lower jaw; below the depression corresponding to the
hyoid bone appear the cartilages of the larynx--the thyroid (pomum
Adami), projecting forwards, and more marked in the male than in the
female, and the cricoid cartilage, to which the windpipe or trachea is
attached. Separating the tongue and larynx from the vertebral column
is the cavity of the pharynx, which is continuous below the cricoid
cartilage, with the gullet or œsophagus, situated behind the windpipe
or trachea. The œsophagus is soft and fleshy, with a cavity effaced
when empty. The _trachea_, which on account of its functions is always
open, is formed of incomplete cartilaginous rings, which give it an
almost cylindrical form and render it prominent below in the middle
line of the neck. Crossing the upper part of the trachea is the median
part of a gland known as the _thyroid body_, which in some cases
(especially in the female) may form a slight rounded projection in the
infra-hyoid region of the neck.
[Illustration: FIG. 88.
MUSCLES OF THE NECK AND FACE.--1, frontal;--2, occipital;--3,
aponeurosis of the occipito-frontalis;--4, temporal;--6,
orbicularis palpebrarum;--7, levator labii superioris et alæ
nasi;--8, dilator naris anterior and posterior;--9, compressor
naris;--9′, pyramidalis nasi;--10, zygomaticus minor;--11,
zygomaticus major;--12, masseter;--13, levator anguli oris;--14,
levator labii superioris;--15, orbicularis oris;--16,
buccinator;--16′, depressor anguli oris, or triangularis
oris;--17, depressor labii inferioris, or quadratus menti;--19,
sterno-cleido-mastoid;--20, trapezius;--21, digastric and
stylo-hyoid;--22, anterior belly of the digastric;--24,
omo-hyoid;--25, sterno-hyoid;--26, thyro-hyoid;--27,
mylo-hyoid;--28, 29, splenius.]
The _infra-hyoid muscles_ ascend from the back of the sternum, inside
the thorax, towards the lower border of the hyoid bone. They are four
in number--two superficial (the omo-hyoid and sterno-hyoid) and two
deep (the sterno-thyroid and thyro-hyoid).
The _omo-hyoid_ (24, Fig. 88; 17, 18, Fig. 89) is a small muscle, long
and slender, with a very remarkable course. It consists of two fleshy
bellies with an intermediate tendon. It arises from the upper border
of the scapula near the root of the coracoid process, and is directed
at first obliquely forwards and inwards behind the clavicle; after
crossing the posterior triangle it turns on itself, and is directed
upwards beneath the sterno-mastoid and through the anterior triangle
to be inserted into the lateral part of the lower border of the hyoid
bone. This muscle, covered at first by the trapezius and then by the
sterno-mastoid, is thus visible on the model in only two parts of
its course--in front of the sterno-mastoid, where its anterior belly
appears over the trachea, and again in the posterior triangle, where
its posterior belly occupies the lower portion of the groove which
separates the trapezius from the sterno-mastoid. Although deeply
placed, this muscle becomes visible beneath the skin, for it is brought
into sharp relief during certain actions. Evidently, from its slender
form, we cannot expect it to be an important elevator of the scapula;
but it serves to depress the hyoid bone. The most important function
of the muscle is by its contraction, especially during spasmodic
efforts in respiration, as in the strong inspiration in sighing or
sobbing; in such cases it prevents the skin and loose aponeurotic
tissue from being too strongly depressed in the supra-clavicular fossa
by atmospheric pressure. Therefore, if the neck is thin and the fossa
well marked, as in an aged female, during the movements of sobbing or
abrupt respiration a cord corresponding to the posterior belly of the
omo-hyoid muscle may be clearly seen in the supra-clavicular fossa.
The _sterno-hyoid_ (25, Fig, 88; 14, Fig. 89) forms a long, thin,
fleshy band, extending from the back of the sternum and clavicle
upwards to the lower border of the hyoid bone. Both these muscles are
contiguous to each other by their internal borders above, but below, at
the deep median fossa corresponding to the episternal notch, they are
separated by a narrow angular space, in which appear the trachea and
the inner border of the sterno-thyroid muscle.
The two deep muscles of this region (the sterno-thyroid and
thyro-hyoid) are continuous with one another. They lie beneath the
sterno-hyoid and omo-hyoid muscles, and cover the trachea and larynx
in the front of the neck. The _sterno-thyroid_ muscle arises from
the posterior part of the first piece of the sternum, and ascends
vertically beneath the sterno-hyoid. It appears at the inner border
of the sterno-hyoid at the lower part of the neck on each side of the
middle line (Fig. 68); reaching the thyroid cartilage, it is inserted
into an oblique line on its external surface. The thyro-hyoid muscle
arising from this oblique line on the thyroid cartilage ascends to be
inserted into the hyoid bone.
[Illustration: FIG. 89.
THE MUSCLES OF THE NECK.--1, digastric (posterior belly);--2,
digastric (anterior belly);--3, hyoid bone;--4, stylo-hyoid;--5,
mylo-hyoid;--6, genio-hyoid;--7, stylo-glossus;--8,
hyo-glossus;--9, internal pterygoid;--10, stylo-pharyngeus;--11,
sterno-cleido-mastoid;--12, sterno-cleido-mastoid (sternal
head);--13, sterno-cleido-mastoid (clavicular head);--14,
sterno-hyoid;--15, sterno-thyroid;--16, thyro-hyoid;--17,
omo-hyoid (anterior belly);--18, omo-hyoid (posterior
belly);--19, trapezius;--20, clavicle;--21, scalenus posticus.]
The _supra-hyoid muscles_ proceed from the hyoid bone to the base of
the skull and lower jaw, and by their contraction elevate this bone,
as can be easily observed in those who bring into action the pharynx
or larynx, as in the act of singing or swallowing. The first of these
supra-hyoid muscles is the _digastric muscle_, composed of two fleshy
bellies connected together by an intermediate tendon (1, 2, Fig. 89).
The _posterior belly_ connects the hyoid bone with the under-surface
of the mastoid process; the anterior belly connects the hyoid bone
to the lower border of the lower jaw near the chin. Next come two
other muscles one behind, lying alongside the posterior belly of the
digastric, and called the _stylo-hyoid_ (4, Fig. 89), and another
in front, called the _mylo-hyoid_ (5, Fig. 89); this muscle lies
underneath the anterior belly, and extends between the hyoid bone
and the deep surface of the body of the lower jaw. It thus forms a
partition closing in the floor of the mouth on each side of the tongue.
The _digastric_ muscle arises from the under-surface of the mastoid
process of the temporal bone. From this origin its fusiform posterior
belly descends obliquely downwards and forwards, and near the hyoid
bone is replaced by a round tendon. This tendon is attached by a
fibrous pulley to the hyoid bone, and, taking its fixed point here, it
turns abruptly, so as to be directed obliquely upwards and forwards
towards the chin; at the same time this tendon is soon replaced by
the anterior belly, also fusiform, which is inserted on the posterior
surface of the symphysis of the chin, into a small depression called
the digastric fossa. We see that this muscle, with its two bellies, is
admirably arranged to raise the hyoid bone, and consequently the whole
of the larynx; for the anterior belly carries the hyoid bone upwards
and forwards, the other upwards and backwards, and if the two contract
at the same time they will raise the hyoid bone directly upwards.
The _stylo-hyoid_ is a small muscular fasciculus which accompanies
the posterior belly of the digastric (21, Fig. 88). Arising from the
styloid process of the temporal bone, this muscle passes downwards and
forwards beneath the posterior belly of the digastric, with which it
is closely connected; at the level of the hyoid bone the tendon of the
digastric pierces the stylo-hyoid, and the latter terminates by an
aponeurotic slip which is inserted into the lateral part of the hyoid
bone. On the model the stylo-hyoid and digastric are united in one
cylindrical bundle (Fig. 88).
The _mylo-hyoid_ muscle (27, Fig. 88) forms the floor of the cavity
of the mouth. It constitutes a quadrilateral fleshy plane, attached
by its upper border to the inner surface of the body of the lower jaw
in a prominent oblique line; by its inferior border it is inserted
into the hyoid bone. Above the hyoid bone the mylo-hyoid muscles are
also inserted in the middle line into a fibrous raphe, so that the two
muscles, that of the right and left side, form practically a single
layer constituting the floor of the mouth. This muscle is covered
on its deep surface by the mucous membrane of the mouth and by the
fleshy fibres of muscles which are not visible externally, and which,
arising from the small tubercles developed on the posterior surface of
the symphysis of the chin (_genial tubercles_), are inserted either
into the hyoid bone (_genio-hyoid_) (6, Fig. 89) or into the tongue
(_genio-hyo-glossus_). The mylo-hyoid muscles, like the previous
muscles, assist in raising the hyoid bone and larynx; it is to be
noted, too, that the supra-hyoid muscles also act in depressing the
lower jaw and opening the mouth.