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© 1998, 1999, WetCanvas! |
[ Home: ArtSchool Online: Basic Anatomy for the Artist: Lesson 5 ]
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© 1998, 1999, WetCanvas! |
The vertebral column has four curves which give the basic rhythm to the body. The cervical curve is composed of seven vertebrae and their discs. It begins to develop late in uterine life and begins to become convex forward when the child begins to hold up its head. The thoracic curve is concave forward, and is composed of twelve vertebrae and is present at birth. The lumbar curve is convex forward and appears when the child begins to walk at about eighteen months. It becomes more prominent in the female than in the male because the fifth lumbar vertebrae articulates with the sacrum of the pelvis which has a more forward tilt in the female. This creates a more backward thrust of the buttocks. The pelvic curve, concave forward, is composed of five sacral vertebrae and the coccyx which has four vestigial tail vertebrae. The sacral vertebrae are fused in the adult and create the flat area seen just below the prominent lumbar curve.
There is greater
action possible in both bending and rotation in the cervical and
lumbar areas of the vertebral column than in the thoracic area.
This is because the cervical and lumbar vertebrae have more generous
articular discs between them. Also, the thoracic vertebrae are
limited in action by the joints which the ribs have with them,
and the very strong fibers at each joint.
In the male figure, the two muscle straps of the rectus abdominis
which are attached to the front of the rib cage and to the front
of the pubis, by contracting, bring these two areas closer together.
In the female figure, the sacrospinales which is attached
the whole length of the back from skull to pelvis, by contracting,
bring the skull and the pelvis closer together.
The Pectoralis
Major is a large fan-shaped muscle on the chest wall. It has
two parts, or heads as they are called, originating from the clavicle
and the rib cage. The clavicular head is attached to one half
of the anterior (front) and medial (towards the midline) part
of the clavicle. The sternal head attaches to the area of the
sterno-clavicular joint, the sternum, and the fifth and sixth
costal cartilages. The muscle is inserted into the top part of
the humerus. When the pectoralis contracts, the humerus being
freer to move than the thorax, changes position. The insertion
has a special feature as the lower muscle bundles of the sternal
head roll under as they pass from the chest to the humerus, in
one twist. This plan provides more mobility for the humerus.
When the arm is relaxed, this whole rolled border can be grasped
in the hand. It creates the front "wing" of the armpit
and is a form which can always be seen. When the arm is brought
forward against resistance the two heads are often seen as separate
entities. The main action it provides is adducting (bringing toward
the midline) the arm.
The biceps (meaning two heads) has two tendinous origins.
The long head is attached to the scapula just above the glenoid
fossa and the short head is attached to the coracoid process.
The long head lies in a special groove in the bone at the top
end of the humerus. It is held in that bicipital groove by a fibrous
ligament which creates a tunnel for it. The long and short heads
meet to form the belly of the biceps, and the insertion of the
muscle is into the tuberosity of the radius. The biceps flexes
(bends toward the body) the elbow joint, and is also the powerful
supinator of the forearm. Because the radius can rotate in its
socket, when the biceps contracts, the radius is pulled over the
ulna and the forearm and arm are turned over.
The tendon of the pectoralis passes on top of the biceps to its
attachment on the far edge (the lateral lip) of the bicipital
groove.
The Rectus Abdominis is composed of two straps attached
above to the xiphoid process and the cartilage of the fifth, sixth,
and seventh ribs. Below it is attached to the front of the pubis.
Each strap has an attachment about 76mm across at the rib cage
and about 25mm across at the pubis. The lateral borders are therefore
diagonal. There are fibrous horizontal intersections in the muscle
because it is of segmental origin. These are at the level of the
xipohoid, the umbilicus, and halfway between. The whole muscle
is enclosed in a sheath, which are sheets of aponeurotic tissue
(flattened tendons) of the waist muscles which will be discussed
later. Above the umbilicus the pair of straps are separated and
the sheath meets in the midline which shows as an indentation
on the surface. It is called the linea alba (white line). Below
the umbilicus the pair of straps are closer together but an indentation
can often be seen here, especially if the muscles are well developed.
When the rectus abdominis contracts it brings the front of the
rib cage and the front of the pelvis closer together. It is used
to raise the body from the lying position to the sitting position
and is a very important muscle. Also, its form is important for
the artist.
The Sartorius is the longest muscle in the body, a narrow
strap, which rises from the anterior superior spine and the bone
below and is inserted by a long flattened tendon into the medial
surface of the upper part of the shaft of the tibia. When it contracts
(by about 15cm) it flexes the hip joint, rotates the thigh laterally
and flexes the knee. When both muscles are working it brings the
legs into the cross-legged position. For the artist, although
its form may not be seen clearly, there is a definite feel of
the muscle as it creates a long diagonal across the front of the
thigh dividing the front aspect of the thigh into two forms.
The Iliopsoas
has two parts. One arises from the inner surface of the hip bone
and the other from the sides and bodies of the lumbar vertebrae
and their discs. It flexes the hip, pulling it toward the trunk.
The muscles pass under the inguinal ligament and attach to the
lesser trachanter of the femur. The femoral artery, vein and nerve
which supply the front of the thigh lie on the front of this muscle
and behind the inguinal ligament. This is a depressed area on
the thigh, to be looked for, and is called the femoral triangle.
The Pectineus, adductor longus, adductor
brevis which lies beneath the longus, and the adductor
magnus all rise from the pubis. They insert into the femur
at the back along the roughened line of bone which they partially
create by their pull on it, called the linea aspera.
The word adductor means to bring or lead to the midline. The adductor
muscles draw the femur to the midline when they contract and can
also draw one thigh over the other. They also flex the thigh by
pulling the femur up toward the pelvis.
For the artist all of these muscles are usually seen as one great
form on the inner region of the thigh. When they are working,
adducting or flexing, the form will thicken and change, in action.
The gracilis is also included with this group adducting
the thigh but as it inserts into the upper part of the medial
surface
of the tibia, it flexes the knee and not the thigh.
When the leg is thrust away from the body (abduction - to lead
away) these muscles can become more delineated as they are when
stretched.
See you next time!