eyes

• Anteriorly, the eyes are protected by the eyelids, which meet at the medial and lateral corners of the eye, the medial and lateral commissure (canthus), respectively.
• Projecting from the border of each eyelid are the eyelashes.
• Modified sebaceous glands associated with the eyelid edges are the tarsal glands;
• tarsal glands. these glands produce an oily secretion that lubricates the eye.
• CILIARY GLANDS, modified sweat glands, lie between the eyelashes.
• A delicate membrane, the conjunctiva, lines the eyelids and covers part of the outer surface of the eyeball.
• The lacrimal apparatus consists of the lacrimal gland and a number of ducts that drain the lacrimal secretions into the nasal cavity.
• The lacrimal glands are located above the lateral end of each eye; they continually release a SALT SOLUTION (tears) onto the anterior surface of the eyeball through several small ducts.
• The tears flush across the eyeball into the lacrimal canaliculi medially, then into the lacrimal sac, and finally into the nasolacrimal duct, which empties into the nasal cavity.
• lysozyme, an enzyme that destroys bacteria;
• lysozyme. it cleanses and protects the eye surface as it moistens and lubricates it.
• extrensic eye muscle are attached to the outer surface of the eye.
• extrinsic eye muscle. these muscles produce gross eye movements and make it possible for the eyes to follow a moving object.
• The outermost layer, called the fibrous layer, consists of the protective sclera and the transparent cornea.
• The sclera, thick, glistening, white connective tissue, is seen anteriorly as the “WHITE OF THE EYE”.
• Cornea. 

  The central anterior portion of the fibrous layer is crystal clear; this “WINDOW” is the cornea through which light enters the eye.
• The middle eyeball of the layer, the vascular layer, has THREE DISTINGUISHABLE REGIONS: the choroid, the ciliary body, and the iris.
• Most posterior is the choroid, a blood-rich nutritive tunic that contains a dark pigment.
• Moving anteriorly, the CHOROID is modified to form two smooth muscle structures, the CILIARY BODY, to which the lens is attached by a suspensory ligament called CILIARY ZONULE, and then the iris.
• The pigmented iris has a rounded opening, the pupil, through which light passes.
• The outer pigmented layer of the retina is composed pigmented cells that, like those of the choroid, absorb light and prevent light from scattering inside the eye.
• The innermost sensory layer of the eye is the delicate two-layered RETINA, which extends anteriorly only to the ciliary body.
• The transparent inner neural layer of the retina contains millions of receptor cells, the rods and cones, which are called PHOTORECEPTORS because they respond to light.
•  Electrical signals pass from the photoreceptors via a two-neuron chain -BIPOLAR CELLS and then GANGLION CELLS
•  The photoreceptor cells are distributed over the entire retina, except where the optic nerve leaves the eyeball; this site is called the OPTIC DISC, or BLIND SPOT.
• Lateral to each blind spot is the fovea centralis, a tiny pit that contains only cones.
•  The lens divides the eye into two segments or chambers; anterior (aqueous) segment and posterior (vitreous) segment.
• the ANTERIOR (AQUEOUS) SEGMENT, anterior to the lens, contains a clear.
• watery fluid called AQUEOUS HUMOR.
• the POSTERIOR (VITREOUS) SEGMENT posterior to the lens,
• a gel-like substance called either VITREOUS HUMOR.
• Vitreous humor helps prevent the eyeball from collapsing inward by reinforcing it internally.
•  Aqueous humor is similar to blood plasma and is continually secreted by a special of the choroid.
• aqueous humor. helps maintain intraocular pressure, or the pressure inside the eye.
• Aqueous humor is reabsorbed into the venous blood through the scleral venous sinus, or canal of Schlemm, which is located at the junction of the sclera
• When the eyes are suddenly exposed to bright light, the pupils immediately constrict; this is the photopupillary reflex.
• Photo pupillary reflex. this protective reflex prevents excessively bright light from damaging the delicate photoreceptors.
• Accommodation pupillary reflex.

   The pupils also constrict reflexively when we view close objects.