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Basic Anatomy for the Artist

Lesson #2

 

MUSCLES AND ACTIONS OF THE EYES

There are six muscles attached to the ball of the eye which then insert into the bone of the orbital cavity.

These muscles are:

-The Superior Rectus
-The Inferior Rectus
-The Medial Rectus
-The Lateral Rectus
-The Superior Oblique
-The Inferior Oblique

They are responsible for all the actions of the eyes, which work together linked by brain pathways.

For example, to look to the right the lateral rectus of the right eye and the medial rectus of the left eye are both contracting. The two oblique muscles work with the rectus muscles to give the diagonal
movements to the eyes.

In drawing the eye remember that the colored iris is a flat disc with a hole in it. When a flat disc is turned away from one it becomes an ellipse. When the eyes are turned up or down, or to either side, the shape of the iris is always slightly elliptical. Only when a person is looking directly at you is the disc a circle. Also the eyelids have a thickness which can be seen clearly. The upper lid often casts a shadow on the eyeball if the light is falling on the lid.

THE FRONT AND SIDE OF HEAD WITH NECK COMPONENTS


The Hyoid Bone is the anchor point for the muscles which open the mouth by pulling the mandible (jaw) down. It is also the attachment point for all but one of the muscles at the front of the neck. It is U-shaped and has a middle, the body, and elongated processes on either side, the lesser and greater horns. It has fibrous attachment to the thyroid cartilage just below it, and thus to the cricoid cartilage and the cartilaginous rings of the trachea (incomplete at the back) which are also joined together by strong fibers. Altogether these form the neck section of the air passage to the lungs. This neck section, especially on thinner necks, is seen as a rounded column and the thyroid cartilage, cricoid cartilage, and the rings of the trachea can at times be seen.

The Thyroid Cartilage is formed by two plates of cartilage which in development have fused together in an angle in the front to make a V-shaped structure. posteriorly, the edges are thickened and have small horn projections above and below. Beneath, and joined to it by both muscle and fibers is the complete
ring of the Cricoid Cartilage. These two cartilages house the components of the vocal cords. In the male, the two plates fuse together at a more acute angle than the female. These plates are also larger and therefore make a more prominent structure in the male neck. In the action of swallowing, the hyoid bone is pulled forward and the thyroid cartilage rides up behind it and assists in shutting off the air passage to block food entering.

THE MANDIBULAR JOINT (JAW JOINT)


The jaw joint is a synovial type of joint which is discussed more fully later in the series. The convex head (condyle) of the jaw fits against a concavo-convex area of the skull to make this joint. This is just in front of the external auditory meatus, the opening into the bony canal of the ear. By feeling on yourself where the jaw joint is in relationship to this opening, the placing of the ear can be established. Both surfaces of bone which are meeting to create the joint are covered by cartilage which is a thin gristly layer of very firm consistency, with a smooth surface. It is a living substance but has no blood or nerve supply, gaining its nutrients from the synovial fluid which fills the joint space.

The jaw joint has a unique disc of cartilage which divides the joint space into lower and upper compartments. This allows a special gliding movement so the head can shift forward in the joint. At this stage a convex surface is meeting another convex surface (when the jaw is depressed and the mouth open) and the round form of the head can be seen in front of the ear, on the surface. Also, when the jaw is moved sideways, the form of the head can be seen and felt on the opposite side to which the jaw is swinging.

By placing the fingers over the joint area, and the thumbs on the angles of the jaw, the head can be felt, and you'll understand better what to look for, in these actions.

MUSCLES CONTROLLING THE ACTION OF THE JAW


The Temporalis is fan-shaped, its fleshy part attaching to the temporal bone on the side of the skull, and its tendons passing under the zygomatic arch to insert into the coronoid process of the jaw. When it contracts it works with the contraction of the muscle bundles of the Masseter, pulling the lower jaw up against the upper jaw, with force, as when the teeth are clinched.

The Masseter (meaning chewer) is a thick quadrangular muscle. It passes diagonally down and back from its attachment to the zygomatic arch at its lower edge, to the lower border of the ramus of the jaw. It is used in chewing along with four deep muscles. The masseter also protrudes the jaw. This muscle has a rich form on one side of the jaw. By putting the fingers against this area and clinching and unclinching the jaw, the contractions and change of form can be felt. The front border of it is often visible, even though it lies beneath the facial muscles.

The pair of Geniohyoids extend from small tubercles on the inner rim of the chin area of the jaw closer to the hyoid bone, as the jaw is freer to move.

The pair of Mylohyoids fill the whole triangle under the jaw. Their muscle bundles run downward and medially to intersect in a midline raphe (seam). They also attach to the upper edge of the hyoid bone, and lie in front of the geniohyoids.

The pair of Digastrics each have two bellies joined by a central tendon. The posterior bellies arise from under the mastoid process of the skull. The tendon joining the two bellies is held down to the hyoid bone by a sling of fascia (fibrous tissue). The anterior bellies are attached to the inner rim of the chin area of the jaw. They lie in front of the mylohoids.

These three muscles constitute the form seen under the chin region. When the neck is stretched backward, there is often a midline indentation caused by the space between the two digastric anterior bellies and the raphe underneath.

These three pairs of muscles act on the hyoid bone to raise and steady it during the act of swallowing. The geniohyoids with the digrastrics depress the jaw by pulling down on the front margin.

LANDMARKS OF THE NECK


The internal jugular vein, the internal and external carotid arteries, the sympathetic nerve trunk, and cranial nerves ten, eleven, twelve lie in the anterior triangle, passing deep to the posterior belly of the digastic.

Running diagonally upward, in the posterior triangle, are the three Scalene muscles, the Levator Scapulae above them, and then the Splenius. The scalenes which are attached to the transverse processes of the cervical vertebrae bend the neck to the side when they contract. They are attached to the first and second ribs as shown above. The splenius bends the neck and rotates the head to the same side, as it has both cervical vertebrae and skull attachments.

See you next time!