e. Muscles of the Eye. The structure of the eye includes the muscles of the
eye--intrinsic muscles and extrinsic muscles. Intrinsic muscles include the two
involuntary muscles within the eye--the iris and the ciliary muscles. The iris regulates
the amount of light which can enter the eye. The iris does this by dilating (opening
more) and by constricting (opening less). The extrinsic muscles are composed of four
rectus muscles (superior muscles, inferior muscles, medial muscles, and lateral
muscles) and two oblique muscles (superior muscles, and inferior muscles). The
extrinsic muscles move the eye within the orbit. The eye is the only organ with both
voluntary and involuntary muscles.
ACCESSORY STRUCTURES OF THE EYE
The site of accessory structures of the eye are as follows: eyebrows, eyelashes,
eyelids, conjunctiva, and lacrimal apparatus.
a. Eyebrows. The eyebrows are made up of several rows of hair above the
upper eyelids. The eyebrows form an arch at the junction of the upper eyelid and the
forehead and structurally resemble the hairy scalp. The hairs are generally coarse and
grow toward the side of the face.
b. Eyelashes. Eyelashes are a row of hairs which project from the border of
each eyelid. Eyelashes coming from the upper eyelid are usually long and turn upward;
eyelashes from the lower eyelid are generally short and turn downward. Sebaceous
glands at the base of eyelash hair follicles pour a lubricating fluid into the follicles.
When these glands become infected, a sty forms.
c. Eyelids. Each eyelid consists of epidermis, dermis, subcutaneous
connective tissue, fibers of oculi muscle, a tarsal plate, tarsal glands, and a conjunctiva.
The epidermis, dermis, and subcutaneous connective tissue make up the skin of the
eyelid. The tarsal plate is a thick fold of connective tissue that forms part of the inner
wall of each eyelid. The tarsal plate gives form and support to the eyelids. The
conjunctiva is a mucous membrane which lines the inner aspect of the eyelids as well
as the visible portion of the cornea (the glossy, transparent part of the cornea).
d. Lacrimal Apparatus. This is the term used for a group of structures that
manufacture and drain away tears. Included are the lacrimal glands, the execratory
lacrimal ducts, the lacrimal canals, the lacrimal sacs, and the nasolacrimal ducts.
(1) Lacrimal gland. A lacrimal gland is located in the upper outer corner of
each orbit. The lacrimal glands secret lacrimal fluid through small ducts into the space
between the external surface of the eyeball and the upper eyelid.