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© 1998, 1999, WetCanvas! |
[ Home: ArtSchool Online: Basic Anatomy for the Artist: Lesson 3 ]
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© 1998, 1999, WetCanvas! |
The
sternomastoid is the most important muscle landmark in
the front and side of the neck as its form is always visible.
It arises by two attachments, from the sternum and the clavicle.
These are called the heads of the sternomastoid. The tendon of
the sternal attachment is seen as a taut band as it arises from
the front and upper border of the Manubrium. The clavicular
head is a wide flat tendon and its lateral edge can usually be
seen.
The two heads blend together to joint a thick form of muscle bundles
to make a heavy strap. This strap becomes flatter as it turns
upward and back to insert into the mastoid process and also the
superior nuchal line at the back of the skull. Remember this is
a thick muscle which can catch light and turn away into shadow.
It is tempting to delineate it by lines on either side which can
deny its form.
Its action, when its cells contract, is to bring the region of
its insertion into the mastoid process and the back of the skull
closer to the sternum. This is possible because the cervical vertebrae
can bend forward or backward, and the head is brought closer to
the more immobile sternal area. The sternomastoids are the most
obvious, and their contractions can be felt the most when the
head is being raised from the pillow.
The Sternum
(the breast bone) has three parts: The Manubrium, the body,
and the Xiphoid process. The manubrium is the large upper
part with a thickened upper border. The sternal ends of the clavicles
articulate with it making the sterno-clavicular joints, and altogether
they create the suprasternal notch. This is a very important landmark
at the root of the neck. Along with the two taut tendons of the
sternomastoid arising from the manubrium it is a very valuable
area for the artist to look at and ponder.
The body of the sternum is composed of four parts which fuse during
development. Sometimes there are apparent ridges where this fusion
has taken place. The whole body is joined to the manubrium by
a moveable joint which acts as a hinge. It allows the body of
the sternum, along with the ribs attached to it, to swing up when
one breaths in. This can be seen on some people and is called
the Sternal Angle of Louis.
The xiphoid process is ossified in the adult and is a small irregular
pointed process sometimes seen. The whole sternum protects the
great vessels of the heart and the heart itself. The upper ten
ribs attach to it by their cartilaginous parts which makes a firm
but flexible unit to allow for the movement of the rib cage, in
breathing.
The suprasternal notch is a stable point to use in taking relative
measurements. By using a midline through the sternum and the notch,
the angle of the neck and head can be judged. In the position
of the female head shown above, taking the midpoint of the sternal
notch as the anchor point, the distance to the right shoulder
edge is approximately the same as to the top of her upper lip.
The Foramen Magnum
is a large hole in the inferior surface of the skull through which
the brain is continuous with the spinal cord. On either side of
this foramen are paired lumps of bone shaped like rockers with
smooth elliptical surfaces which are convex. These two convex
surfaces articulate with two concave elliptical surfaces on the
superior surface of the first vertebra, the Atlas. The
action possible because of this design is to allow the head to
"rock" on the neck in the nodding action to give the
"yes" answer. The paired lumps of bone on the skull
are called the Occipital Condyles.
The second cervical vertebra is called the axis. From its vertebral
body there is a peg of bone called the Dens, which projects
upward to lie against the atlas and articulate with it. It is
held there by a ligament. This dens allow the atlas to pivot,
and thus create the "no" answer. These are important
and unique actions of the head in its relationship with the neck.
The Ligamentum Nuchae is the continuation in the neck of
the fibrous ligaments which tie the vertabrae together. It is
a strong thin sheet in the midline at the back of the neck, attaching
to the skull and the spines of the seven cervical vertebra. Two
muscles, the splenius and the trapezius are attached to it. A
linear indentation is often seen in the midline running up the
back of the neck. It is caused by the muscles pulling on the edge
of the ligamentum nuchae, the muscles contracting and mounding
up on either side of it.
The Pectoral Girdle
consists of two bones, the clavicle and the scapula. The clavicle
is S-shaped, convex forward in its rounded medial two-thirds,
and concave forward in its remaining third, which is flattened.
The medial end is enlarged and makes a joint with the manubrium
called the Sterno-clavicular joint. The lateral end makes
a joint with the acromion process of the scapula. The acromion
is the large flat free end of the spine of the scapula, which
projects out to the point of the shoulder.
The form of the clavicle with its rounded and flattened parts
can usually be seen as the clavicle lies just under the skin (subcutaneous).
Its enlarged head making its joint with the manubrium is also
a landmark. On the top of the shoulder both the end of the clavicle
and the flattened surface of the acromion have visible forms as
they are subcutaneous.
In the above drawing, the humerus is included with its head making
a joint with the articular shallow cup, the glenoid fossa,
of the scapula. When the arm is raised the scapula rotates. Its
lower point, the inferior angle, swings forward up and around
the rib cage. The glenoid fossa with the head of the humerus rise,
and the acromion and lateral end of the clavicle also. The clavicle
and the acromion can be located redily on yourself, while they
are at rest, and during movement, and their forms studied on yourself
in a mirror.
Action takes place
in the body because the bones of the skeleton make joints (articulate)
with each other, and because muscles act on these joints. There
are three main classes of joints: Fibrous, Cartilaginous,
and Synoval.
FIBROUS joints are those in which two bones are joined together
by fibrous tissue which is composed of strands of non-living material
with cells interspersed and, at times, elastic fibers. This plan
exists where movement is not desired but a little flexibility
may be needed.
CARTILAGINOUS joints exist between the vertebrae of the vertebral
column. Here a pliable fibro-cartilaginous cushion is present
called a vertebral disc. It is a shock absorber, and it is covered
by a capsule of layers of diagonal fibers. This allows the vertebrae
to "squeeze down" a little on the softer disc so the
vertebral column can bend. The diagonal fibers of the capsule
which are attached to the vertebrae above and below the disc strengthen
the joint and also allow for rotation.
SYNOVIAL joints are those in which the articulating ends of two
bones are contained within a capsule which contains synovial lubricating
fluid. The main joints of the body which allow greater movement
are in this class. They include the shoulder joint (ball and socket),
the hip joint (ball and socket), the elbow
joint (hinge and pivotal), the knee joint (hinge), and others.
See you next time!