Anatomy can be painful for some (personally, I hated anatomy in medical school) so I’m going to keep this simple. Eyelids. The eyelids protect and help lubricate the eyes. The eyelid skin itself is very thin, containing no subcutaneous fat, and is supported by a tarsal plate.
This tarsal plate is a fibrous layer that gives the lids shape, strength, and a place for muscles to attach. Underneath and within the tarsal plate lie meibomian glands. These glands secrete oil into the tear film that keeps the tears from evaporating too quickly. Meibomian glands may become inflamed and swell into a granulomatous chalazion that needs to be excised. Don’t confuse a chalazion with a stye. A stye is a pimple- like infection of a sebaceous gland or eyelash follicle, similar to a pimple, and is superficial to the tarsal plate. Styes are painful, while chalazions are not.
Eyelid Movement. Two muscles are responsible for eyelid movement. The orbicularis oculi closes the eyelids and is innervated by cranial nerve 7. Patients with a facial nerve paralyses, such as after Bell’s Palsy, can’t close their eye and the eye may need to be patched (or sutured closed) to protect the cornea.
The levator palpebrae opens the eye and is innervated by CN3. In fact, a common surgical treatment for ptosis involves shortening the levator tendon to open up the eye.
In anatomy and physiology, a duct is a circumscribed channel leading from an exocrine gland or organ.
The anatomy of the eye is complex. The main structures of the eye include: Cornea: clear tissue in the very front of the eye; Iris: colored part of the eye surrounding the pupil; Pupil: dark hole in the. Human Anatomy at EnchantedLearning.com. EnchantedLearning.com is a user-supported site. As a bonus, site members have access to a banner-ad-free version of the site, with print-friendly pages.
Anatomy Review The lists below reflect the anatomical parts within the system that need to be known. To see a picture or a short description of something you see on the list before clicking, test yourself and try to visualize. HI-1014 - Anatomy & Physiology Course Introduction 10m Course Introduction Chapter 01 - Medical Terminology 24m Medical Terminology Word Analysis History of Medical Terms The Process The Approach Translating Medical Terms. BASIC ANATOMY AND PHYSIOLOGY OF THE HUMAN VISUAL SYSTEM Corina van de Pol The human eye is a complex structure designed to gather a significant amount of information about the environment around us. It is the sensor used by.
Human Corneal Anatomy Redefined A Novel Pre-Descemet's Layer (Dua's Layer) Presented at: the Societa Italiana Cellule Staminali e Superficie Oculare, VI CONGRESSO S.I.C.S.S.O. Lecce, June 14–16, 2007; The Royal College of. Schematic diagram of the human eye showing the cornea as separated from the sclera by the corneal limbus.
CN 3 opens the eye like a pillar. CN 7 closes like a fish- hook. Conjunctiva. The conjunctiva is a mucus membrane that covers the front of the eyeball. When you examine the “white part” of a patient’s eyes, you’re actually looking through the semi- transparent conjunctiva to the white sclera of the eyeball underneath. The conjunctiva starts at the edge of the cornea (this location is called the limbus).
It then flows back behind the eye, loops forward, and forms the inside surface of the eyelids. The continuity of this conjunctiva is important, as it keeps objects like eyelashes and your contact lens from sliding back behind your eyeball.
The conjunctiva is also lax enough to allow your eyes to freely move. There is a thickened fold of conjunctiva called the semilunar fold that is located at the medial canthus – it is a homolog of the nictitating membrane seen on sharks. Tear Production and Drainage.
The majority of tears are produced by accessory tear glands located within the eyelid and conjunctiva. Tears flow down the front of the eye and drain out small pores, called lacrimal punctum, which arise on the medial lids.
These puncta are small, but can be seen with the naked eye. After entering the puncta, tears flow down the lacrimal tubing and eventually drain into the nose at the inferior turbinate.
This explains why you get a runny nose when you cry. In 2- 5% of newborns, the drainage valve within the nose isn’t patent at birth, leading to excessive tearing. Fortunately, this often resolves on it’s own, but sometimes we need to force open the pathway with a metal probe. Eyedrops meant for local effect, such as beta- blockers, can have impressive systemic side effects when absorbed through the nose. Patients can decrease nasal drainage by squeezing the medial canthus after putting in eyedrops. They should also close their eyes for a few minutes afterwards because blinking acts as a tear pumping mechanism.
The Eyeball: The eyeball is an amazing structure. It is only one inch in diameter, roughly the size of a ping- pong ball, and is a direct extension of the brain. The optic nerve is the only nerve in the body that we can actually see (using our ophthalmoscope) in vivo.
The sclera is white, fibrous, composed of collagen, and is actually continuous with the clear cornea anteriorly. In fact, you can think of the cornea as an extension of the sclera as they look similar under the microscope. The cornea is clear, however, because it is relatively dehydrated. At the back of the eye, the sclera forms the optic sheath encircling the optic nerve. The eyeball is divided into three chambers, not two as you might expect. The anterior chamber lies between the cornea and the iris, the posterior chamber between the iris and the lens, and the vitreous chamber extends from the lens back to the retina. The eye is also filled with two different fluids.
Vitreous humor fills the back vitreous chamber. It is a gel- suspension with a consistency similar to Jell- O. When this occurs, the vitreous can fall in upon itself – usually a harmless event called a PVD (posterior vitreous detachment). This is a watery solution with a high nutrient component that supports the avascular cornea and lens.
Aqueous is continuously produced in the posterior chamber, flowing forward through the pupil into the anterior chamber, where it drains back into the venous circulation via the Canal of Schlemm. The Cornea: The cornea is the clear front surface of the eye. The cornea- air interface actually provides the majority of the eye’s refractive power. The cornea is avascular and gets its nutrition from tears on the outside, aqueous fluid on the inside, and from blood vessels located at the periphery. The outside surface layer is composed of epithelial cells that are easily abraded. Under this lies Bowman’s layer and then the stroma. The corneal stroma makes up 9.
The next layer is Descemet’s membrane, which is really the basal lamina of the endothelium, the final inner layer. The inner endothelium is only one cell layer thick and works as a pump to keep the cornea dehydrated. Endothelial cell count is very important as these cells don’t regenerate when destroyed – the surviving endothelial cells just get bigger and spread out. This is where aqueous is drained, and blockage of this pathway/angle will become important as we discuss glaucoma. The Uvea: The iris, ciliary body, and the choroid plexus are all continuous with each other and are collectively called the uvea. The iris is the colored part of the eye and its primary function is to control the amount of light hitting the retina.
Sympathetic stimulation of the pupil leads to pupil dilation and parasympathetic stimulation leads to constriction. In other words, if you see a bear in the woods, your sympathetics kick in, and your eyes dilate so you can see as much as possible as you run away.
I’ll be using this mnemonic/metaphore many times throughout this book to help you remember this concept. The inner iris flows back and becomes the ciliary body. The ciliary body has two functions: it secretes aqueous fluid and it controls the shape of the lens. The ciliary body contains sphincter muscles that change the lens shape by relaxing the zonular fibers that tether to the lens capsule. The choroid supplies nutrition to the outer one- third of the retina which includes the rod and cone photoreceptors. Retinal detachments can separate the retina from the nutritious choroid, which is disastrous for the photoreceptors as they quickly die without this nourishment. FUN FACT: An ostrich’s eye is larger than its brain.
Lens: The lens sits behind the iris. The lens is unique in that it doesn’t have any innervation or vascularization. The capsule is thin with a consistency of saran wrap and holds the rest of the lens in place. The middle layer is called the cortex, while the central layer is the hard nucleus. Cataracts are described by where they occur such as nuclear cataracts, cortical cataracts, and subcapsular cataracts.
With cataract surgery the outer capsule is left behind and the artificial lens is placed inside this suporting bag. The capsule is held in place by suspensory ligaments called zonules that insert around the periphery and connect to the muscular ciliary body. Histologically, many cell layers can be seen, but they are not worth memorizing at this point. The important ones include the photoreceptor layer, which is located further out (towards the periphery), and the ganglion nerve layer which lies most inward (toward the vitreous). For light to reach the photoreceptor it has to pass through many layers. After light reaches the photoreceptors the visual signal propagates back up to the ganglion nerves.
These ganglion nerves, in turn, course along the surface of the retina toward the optic disk and form the optic nerve running to the brain. The macula is the pigmented area of the retina that is responsible for central vision. Within the central macula lies the fovea, which is a small pit that is involved with extreme central vision. The fovea is very thin and derives its nutrition entirely from the underlying choroid, making it susceptible to injury during retinal detachments.
The optic disk is the entry and exit point of the eye. The central retinal artery and vein pass through here, along with the the ganglion nerves that form the optic nerve. The Orbital Walls: Seven different bones form the orbital walls: don’t be intimidated by this complexity, however, as these bones are not that confusing when you break them down.
The medial wall is a little more complex, however, but is mainly formed by the lacrimal bone (the lacrimal sac drains tears through this bone into the nose) and the ethmoid bone. Sinus infections can erode through this “paper- thin wall” into the orbital cavity and create a dangerous orbital cellulites. The Apex: Entrance into the Orbit. The orbital apex is the entry point for all the nerves and vessels supplying the orbit.
The superior orbital fissure lies between the wings of the sphenoid bones, through which many vessels and nerves pass into the orbit. These muscles insert at the sclera, behind the limbus, and each pull the eye in the direction of their attachment. The superior, medial, and inferior rectus muscles are all controlled by the oculomotor nerve (III). The superior oblique also originates in the posterior orbit, but courses nasally until it reaches the trochlea (or “pulley”) before inserting onto the eye. The inferior oblique originates from the orbital floor and inserts behind the globe near the macula.