Eye Accessory Structures & Muscles

Accessory Structures

• Eyebrows shade the eyes, prevent sweat from entering
• Eyelids protect the eyes and spread lacrimal fluid while blinking
• Tarsal glands in tarsal plate, secrete an oily substance to lubricate the eye
• Ciliary glands are sweat glands between eyelash follicles
• Conjunctivae is a mucous membrane lining the eyes that lubricates them
• Palpebrae refers to the eyelids
• Bulbare refers to the eye
• Medial and lateral commissures are the corners of the eyes
• Medial commissures contain the lacrimal caruncle, which produces a whitish, oily secretion
• Lacrimal glands secrete lacrimal fluid, consisting of mucous, antibodies, and enzymes
• The flow of lacrimal fluid goes from the lacrimal gland to the lacrimal puncta, then to the lacrimal canaliculi, the lacrimal sac, and finally the nasolacrimal duct, which leads to the nasal cavity

Eye Muscles

• Lateral rectus muscle moves the eye laterally
• Medial rectus muscle moves the eye medially
• Superior rectus muscle moves the eye medially and elevates it
• Inferior rectus muscle moves the eye medially and depresses it
• Inferior oblique muscle elevates the eye and enables lateral movement
• Superior oblique muscle depresses the eye and allows lateral movement

External Eye Features

• The fibrous layer is an external feature of the eye
• The sclera is white connective tissue that shapes the eye and provides attachment for eye muscles
• The cornea is continuous with the sclera, transparent, and refracts light

Middle (Vascular) Layer of the Eye

• The choroid is a blood vessel-rich, dark membrane that nourishes the eye
• Melanin in the choroid absorbs excess light
• The ciliary body is a modification surrounding the lens
• The ciliary muscle is smooth muscle that alters the shape of the lens
• The ciliary process is folds of ciliary muscle that form aqueous humor by filtering plasma
• Ciliary zonule is a fine fiber that attaches the ciliary process to the lens
• The pupil is a round opening in the iris through which light enters
• The iris is the anterior portion of the vascular layer with two pigmented layers of smooth muscle controlling the light entering the eye
• The sphincter pupillae constricts the pupil
• The dilator pupillae dilates (expands) the pupil

Inner Layer of the Eye

• The inner layer of the eye is also known as the retina
• The pigmented layer (outer) is a single layer of melanocytes that absorb light and store vitamin A
• The neural layer (inner) has three main types of neurons: photoreceptors, bipolar cells, and ganglion cells
• Photoreceptors (rods and cones) convert light into action potentials

Eye Tests

• Accommodation refers to the eye's ability to focus on objects less than 20 feet away
• The near point is usually about 10 cm from the eye, but lens elasticity decreases with age, so the near point moves further away (presbyopia)
• Visual acuity is tested with the Snellen eye chart at 20 feet
• Reading line 20 at 20 feet away indicates 20/20 vision
• Myopia is nearsightedness where the eyeball is too long, corrected with a concave lens
• Hyperopia is farsightedness where the eyeball is too short, corrected with a convex lens
• Astigmatism results from irregular curvatures of the eyeball
• Color blindness is tested using Ishihara's color plates, identifying deficiencies in cone cells (3 types)