Anatomy & Physiology I and II

Anatomy & Physiology Study Guide

©2018 Achieve Test Prep Page 190 of 367 , are a continuation of the skin. Their continual blinking keeps the surface of the eye lubricated, and they act like windshield wipers, removing dust and debris. The eyelids can also close firmly to protect the delicate surface of the eye. The eyelashes, located along the margins of the eyelids, are hairs that aid to prevent foreign matter (including insects) from touching the eye. Unusually large sebaceous glands are associated with eyelashes. Along the inner margin of the lid, modified sebaceous glands, called tarsal glands or Meibomian glands, secrete a lipid-rich product that helps keep the eyelids from sticking together. The skin covering the observable surface of the eyelid is fine. Deep in the skin lie the muscle fibers of the orbicularis oculi and levator palpebrae superioris muscles; they are responsible for closing the eyelids and raising the upper eyelid, respectively. The epithelium covering the inner surfaces of the eyelids and the outer surface of the eye is called the conjunctiva . It is a mucous membrane protected by a specialized stratified squamous epithelium. The ocular conjunctiva extends to the edges of the cornea, a transparent part of the outer fibrous layer of the eye. The cornea is covered by a very delicate squamous corneal epithelium, five to seven cells thick. A continuous supply of fluid flows over the surface of the eyeball, keeping the ocular conjunctiva and cornea moist and clean. Conjunctivitis, or pinkeye, results from damage to and irritation of the conjunctival surface. The Lacrimal Apparatus The lacrimal apparatus produces, secretes, and removes tears. This apparatus of each eye consists of a lacrimal gland and ducts, lacrimal sac, paired lacrimal canaliculi, and a nasolacrimal duct. The pocket created where the palpebral conjunctiva becomes continuous with the ocular conjunctiva is known as the fornix of the eye. The lateral part of the superior fornix receives 10–12 ducts from the lacrimal gland , or tear gland. The lacrimal gland produces about one mL of tears each day. Blinking sweeps the tears across the ocular surface, and they accumulate at the medial canthus in an area known as the lacrimal lake, or “lake of tears.” The lacrimal puncta (singular, punctum), two small pores, drain the lacrimal lake. They empty into the lacrimal canaliculi, which are small canals that go to the lacrimal sac. From the inferior part of the lacrimal sac, the nasolacrimal duct travels through the nasolacrimal canal, formed by the lacrimal bone and the maxillary bone. The nasolacrimal duct delivers tears to the nasal cavity on that side. The Eye Each eye is a slightly irregular spheroid with an average diameter of 24 mm (almost one inch) and a weight of about 8g (0.28 oz.). The eyeball shares space with the accessory structures of the eye within the orbit (this includes extrinsic eye muscles, the lacrimal gland, and the cranial nerves and blood vessels that supply the eye and adjacent portions of the orbit and face). The eye’s wall consists of three layers, or tunics: an outer fibrous tunic, an intermediate vascular tunic, and an inner neural tunic (retina). The visual receptors, or photoreceptors, are located in the Accessory Structures of the Eye The accessory structures of the eye contain the superficial epithelium of the eye, the eyelids, and the lacrimal structures associated with the production, secretion, and removal of tears. Eyelids and Superficial Epithelium of the Eye The eyelids, or palpebrae