Anatomy & Physiology

Anatomy & Physiology Study Guide 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 neural tunic. The eyeball is hollow; its interior can be divided into two cavities. The large posterior cavity is also called the vitreous chamber. The smaller anterior cavity is subdivided into the anterior and posterior chambers. The form of the eye is stabilized in part by the vitreous body and the clear aqueous humor, which fills the anterior cavity. The Fibro s Tunic The fibrous tunic consists of the sclera and the cornea. The fibrous tunic provides mechanical support and some degree of physical protection, serves as an attachment site for the extrinsic eye muscles, and contains structures that assist in the process of focusing. Most of the ocular surface is protected by the sclera , or “white of the eye,” which consists of a dense fibrous connective tissue containing both collagen and elastic fibers. The six extrinsic eye muscles insert on the sclera. Small blood vessels and nerves from the surface penetrate the sclera to reach internal structures. The vessels are visible as red lines against the white background of collagen fibers. The cornea does not contain blood vessels; the tears that flow across the free surfaces of superficial epithelial cells provide oxygen and nutrients. The cornea also has numerous free nerve endings, and it is the most sensitive portion of the eye. The Vascular Tunic (Uvea) The vascular tunic, or uvea, contains numerous blood vessels, lymphatic vessels, and the intrinsic (smooth) muscles of the eye. This middle layer has many functions. These include providing a route for blood vessels and lymphatics that supply tissues of the eye, controlling the shape of the lens, regulating the light that enters the eye, and secreting and reabsorbing the aqueous humor as it circulates within the chamber. The vascular tunic includes the iris, the ciliary body, and the choroid. The iris contains blood vessels, pigment cells, and two layers of smooth muscle fibers called pupillary muscles. When these muscles contract, they change either the diameter of the pupil or central opening of the iris. Both muscle groups responsible for the contraction of the pupil are controlled by the autonomic nervous system. Achieve Page 191 of 368 ©2018