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ARS VETERINARIA, Jaboticabal, SP, v.29, n.1, 042-051, 2013. ISSN 2175-0106

MORPHOLOGY OF OCULAR SYSTEM OF DOMESTIC ANIMALS

MORFOLOGIA DO SISTEMA OCULAR DOS ANIMAIS DOMÉSTICOS

A. G. B. LEITE1; D. OLIVEIRA1; S. M. BARALDI-ARTONI2

SUMMARY

A literature review on the structures that compose the ocular system of domestic animals is presented in this study. The eyeball is a specialized organ whose primary function is to capture and focus the light onto the photosensitive . It comprises three concentrically arranged layers: an outer layer or fibrous tunic, containing the and ; a middle coat or vascular layer, comprising the , and ; and the inner coat or nervous layer, which is formed by the retina. In addition, there are the and the structures that protect and move the eyeball, such as the orbital fascia, ocular muscles, , and . Each component can be affected by various diseases, which the veterinarian should be able to intervene in order to preserve this important sense organ.

KEY-WORDS: Eyeball. Veterinary Ophthalmology. Vision.

RESUMO

ão de literatura sobre as estruturas que compõem o sistema ocular Este trabalho teve como objetivo realizar uma revis ésticos. O bulbo ocular é um órgão especializado cuja função primária consiste dos animais dom em captar e focalizar a ível. É formado por três túnicas dispostas concentricamente: a camada externa ou túnica luz sobre a retina fotossens órnea; a camada média ou túnica vascular, constituída pela coróide, pelo corpo fibrosa, que consiste em esclera e c ciliar íris; e a camada interna ou túnica nervosa, que é formada pela retina. Além desses envoltórios, há o cristalino ou e pela ão as fáscias orbitárias, os músculos oculares, as lente e as estruturas que protegem e move o bulbo do olho, que s álpebras, ções diversas, as quais o p a conjuntiva e o aparelho lacrimal. Cada componente pode ser acometido por afec édico veterinário deve estar apto a intervir, a fim de preservar esse importante órgão do sentido. m

PALAVRAS-CHAVE: ária. ão. Bulbo ocular. Oftalmologia veterin Vis

INTRODUCTION while bringing great advances in the field (TRAMONTIN, 2010). There are many records addressing the origin of disease is relevant to those who are veterinary ophthalmology and throughout history, man interested in veterinary ophthalmology since the eye has been expressing concern regarding vision. At each exam is the direct observation of its rough condition, development stage of science, interpreted with peculiar even if the ophthalmoscope is a microscope with low religious involvement, the study of the vision had great capacity to magnify the structures that make up the scientific importance (TRAMONTIN, 2010). (SLATTER, 2005). According to Lovato The visual system is the sensory organ that the et al. (2005), the eye exam is important since many animal should endanger less, and is commonly ocular disorders can lead to irreversible loss of sight involved in various clinical diseases (CUNNINGHAM, and impaired aesthetics. 2004). Over the past 300 years, new discoveries and A complete ophthalmic examination can help in published studies allowed better eye examinations establishing a fast and accurate diagnosis for many

1 êmica de Garanhuns, UFRPE Unidade Acad . 2 ências Agrárias e árias – Faculdade de Ci Veterin Unesp. 42 ophthalmic and systemic diseases because the eye can The fibrous tunic consists of sclera and cornea, be visualized up to the level of the posterior chamber which are in the limbus, and is composed of very dense where the is partially exposed (GELATT, 2003). collagen tissue that, resisting the internal pressure, However, Lovato et al. (2005) highlighted the gives firmness and shape to the eye (DYCE et al. importance of primary health care that may assist the 1997). ophthalmologist in identifying diseases in early stages, The middle layer (vascular tunic) consists of the facilitating early intervention with therapeutic choroid, the ciliary body and the iris (JUNQUEIRA & measures for eye recovery. CARNEIRO, 2004). It is also known as and is According to Tramontin (2010), technological situated deeply to the sclera, where is fixed (DYCE et advances increasingly provide new techniques and al. 1997). discoveries in medicine, so further development of The inner layer (tunic nervosa) or retina is the ophthalmology is expected over the years. innermost layer and communicates with the brain by This study presents a literature review on the the optic . In addition to these wrappers, the eye morphology of the structures that make up the visual has the lens, a transparent, biconvex structure which is system of domestic animals. held in position through a circular ligament, the ciliary zonule, which is inserted over a thickened middle layer that covers partially the iris (JUNQUEIRA & LITERATURE REVIEW CARNEIRO, 2004). The visual system captures and focuses the light This work addresses the structures that make on the photoreceptors, transducers that convert light up the ocular system of domestic animals, such as into electrical impulses sending it through the visual dogs, cats, horses, cattle, goats, sheep and pigs, cortex, where the sensation of vision occurs. describing anatomically and histologically the three Homeostatic and anatomical mechanisms, which refine tunics that compose the eyeball and its attachments. and protect the system, vary among species, depending The fibrous tunic, which consists of the cornea and on their functional needs (SLATTER, 2005). sclera; the vascular tunic, composed of the choroid, The electrical information generated by ciliary body and iris; and the nervous tunic, which is exposure of photoreceptors to light undergoes initial formed by the retina are described. Additionally, the neural processing within the retina. Lastly, this lens, the orbital fascia, ocular muscles, eyelids, information leaves the eye through the optic nerve, in conjunctiva and lacrimal apparatus are also described. whose fibers are the axons of retinal ganglion cells.

Most axons of the optic nerve make synapse in the EYE thalamus, and from there the visual information goes to the primary visual cortex in the occipital lobe of the The eye is an organ whose primary function is brain for conscious perception (FRANDSON et al., to collect and focus light on the photosensitive retina. It 2005). lies within a cone-shaped cavity on the face, the orbit, The structures that protect and move the eyeball which houses the eyeball and various other soft tissue include orbital fascia, ocular muscles, the eyelids, structures and ocular adnexa, such as muscles and conjunctiva, and the lacrimal apparatus; most of these glands, which act on the functional eyeball structures are contained within the orbit (DYCE et al., (FRANDSON et al., 2005). The eye shape is 1997). essentially spherical due to the pressure generated by ORBIT the aqueous humor and lack of elasticity of the cornea and sclera (CUNNINGHAM, 2004).

The eye develops from the neural tube ectoderm Orbit is the bone structure of the skull that (neuroectoderm), mesoderm and surface ectoderm. The contains the eye or ocular structures and their neuroectoderm gives rise to the retina and optic nerve surrounding soft tissue (BROOKS, 2005). Domestic while the mesoderm forms the remaining structures, animals display two orbital patterns, incomplete, like in except the lens, lacrimal glands and the epithelium of dogs and cats and complete, like in horses, cattle, goats the conjunctival sac and eyelids, which are derived and pigs (Figure 1). The orbit separates the eye from from the surface ectoderm (DYCE et al., 1997). the cranial cavity while foramina and fissures on the According to Dyce et al. (1997), the main walls determine the pathways of blood vessels and irrigation of the eye is performed by the external from the brain to the eye (GELATT, 2003). ophthalmic artery, a branch derived from the maxillary The orbit consists of frontal, lacrimal, sphenoid, artery, ventral to the orbit, to irrigate structures rostral zygomatic, palatine and bone. Three bone to the face. The arteries that originate from the processes, the zygomatic process of the , ophthalmic artery can be divided into the following the frontal process of the and the three groups: those that irrigate the eyeball, those that zygomatic process of the temporal bone, play a role in irrigate the eye muscles and those that leave the orbit to the formation of the lateral bone wall of the orbit irrigate adjacent structures. (GETTY, 1986). The eye consists of three tunics concentrically The location of the orbits inside the skull arranged: the outer layer (fibrous tunic), the middle determines the degree of binocular vision. Animals layer (vascular tunic) and the inner layer (tunica with laterally placed orbits, like horses, have a reduced nervosa) (JUNQUEIRA & CARNEIRO, 2004). binocular vision and depth perception in relation to 43

ARS VETERINARIA, Jaboticabal, SP, v.29, n.1, 042-051, 2013. ISSN 2175-0106

Figure 1 - Dog skull (A) showing the incomplete orbit (arrow) and cattle skull (B), which illustrates the orbit (arrow). Source: SA, 2012. Laboratory of Experimental Ophthalmology - UFRPE.

animals with more frontally placed, like cats and dogs (BROOKS, 2005). The limbus is the transition zone between the Unlike the human orbits, which are conical, cornea and sclera. The homogeneous and transparent four-sided pyramidal cavities, the ventral portion of the corneal collagen becomes opaque and fibrous orbit of the domestic species is limited by soft tissues, (JUNQUEIRA & CARNEIRO, 2004). The limbus, notably the pterygoid muscles (FRANDSON et al., cornea, sclera, conjunctiva and the episcleral blade are 2005). According to Brooks (2005), the orbit area overlapped and adhered in the region where they meet behind the ocular bulb is called retrobulbar. The (VIETH et al., 1995). In this highly vascularized zone, periorbital area indicates the area around the orbit, there are blood vessels that play an important role in including the eyelids, soft tissue and skull. inflammatory processes of the cornea (JUNQUEIRA & CARNEIRO, 2004). SCLERA According to Brooks (2005), limbal stem cells are stimulated to form the during

The sclera is the major part of the fibrous the corneal ulceration. Inflammatory cells and blood tunic of the eye (SLATTER, 2005). It is white, vessels penetrate the cornea through the limbus. sometimes gray or blue, and meets the cornea in a transitional region called limbus (FRANDSON et al., CORNEA 2005). It is tough and elastic since its main function is to protect the intraocular contents (VIETH, et al., 1995; The cornea is the transparent front layer of the URBAN et al., 2002). fibrous tunic and the most powerful refractory layer of The axons of the optic nerve pierce the sclera the eye. Transparency and regular curvature are at the posterior pole of the eye, the cribriform area. The essential to focus light on the retina (FRANDSON et sclera is the insertion site of the of al., 2005). Because it is avascular; it receives nutrients the eye (FRANDSON et al., 2005) while innervation through the vessels of the limbus and the fluid from the occurs due to long and short posterior ciliary nerves, anterior chamber (JUNQUEIRA & CARNEIRO, trigeminal branches (URBANO et al., 2002). 2004). According Dyce et al. (1997), the sclera is Despite careful distribution of the fibers, perforated by several small ciliary arteries and larger corneal transparency is not only a structural vorticose veins. It is covered by a thin membrane, phenomenon but also depends on the physiological and sheath bulb, separating it from the retrobulbar fat, continuous pumping of interstitial fluids, a process providing a cavity where the bulb can move. located in the posterior epithelium (DYCE et al., The sclera layers are episclera, stroma and 1997). According to Turner (2010), the corneal lamina fusca. The episclera is composed of dense endothelium has pumps to remove excess fluid, which vascular connective tissue that originates from the leaves the cornea relatively dehydrated and also helps surface scleral stroma and episcleral blade that covers to maintain transparency. If this pump fails, dense it. The scleral stroma consists of collagen fibers corneal edema may occur, leading to bluish-gray arranged in an uneven checkerboard pattern parallel to coloration of the cornea and increasing thickness. the surface of the . The lamina fusca innermost A cross-section of the cornea shows five distinct layer is in close contact with the uvea and presents regions: the tear film; the corneal epithelium, which is pigmented cells (URBAN et al. 2002). non-keratinized stratified squamous and its basement membrane; the corneal stroma, which is composed of LIMBUS fibrocytes, keratocytes, collagen and ground substance;

44 the Descemet's membrane, which is the endothelial fiber bundles which insert in the sclera on one side and basement membrane; and, the , in different regions of the ciliary body, on the other which has the thickness of a cell, and is located side. One of these bundles has the function of ’s membrane (SLATTER, subsequent to the Descemet distending the choroid while the other, when 2005). contracted, relaxes the tension of the lens The corneal surface is very sensitive, as it is (JUNQUEIRA & CARNEIRO, 2004). densely innervated with sensory nerve fibers close to the anterior epithelium. These innervations emerge IRIS from the long ciliary nerves, which are branches of the ophthalmic nerve. Their axons form the afferent branch The iris is the most anterior portion of the of the corneal reflex, which closes the lids when the vascular tunic (VIETH et al., 1995), it consists of a cornea is touched (DYCE et al., 1997). pigmented ring that sits between the cornea and the lens, whose primary function is to control the light CHOROID entering the eye through the (FERRAREZI, 2010). According to Brooks (2005), pupil size changes The choroid is an intermediate layer located as the intensity of the ambient light varies, constricted between the sclera and retina (VIETH et al., 1995), and dilated in bright and low light, respectively, thus which is at the rear of the vascular tunic. It is highly controlling the amount of light entering the eye. vascularized and has multiple layers. The deepest layer The pupillary edge has a smooth sphincter tapetum, is the which is a reflective surface designed to muscle responsible for the contraction of the pupil, and pop up the light entering in the retina, thus increasing a myoepithelial structure, radially arranged, which the dim light vision (FRANDSON et al., 2005). helps to dilate the pupil. The iris motor innervation is The loose connective tissue between choroidal autonomous, with adrenergic and cholinergic fibers vessels is rich in cells, collagen and elastic fibers. Cells innervating both the sphincter and the dilator muscle of containing melanin pigment, which confer dark color the pupil, although the first is assigned the to this layer, are frequently observed (JUNQUEIRA & predominance of a parasympathetic innervation and the CARNEIRO, 2004). The vessels network in the second, to the maintenance of the tonicity, a choroid is irrigated by the posterior ciliary arteries and sympathetic innervation (BICAS, 1997). drained by vorticose veins (DYCE et al., 1997) while According to Dyce et al. (1997), the iris is a flat the choroid is responsible for feeding the external part ring of tissue attached, at its periphery, to the sclera by of the retina (VIETH et al., 1995). the pectineal ligament and ciliary body. The iris In the dorsal part of the eye, the choroid forms a divides the space between the cornea and the lens in slightly reflective area of varied coloration, known as the anterior and posterior chambers, respectively, tapetum . The cells contain crystalline which communicate through the pupil. Both are filled rods arranged so that light falling on them is divided by the aqueous humor. The place where the anterior into its components, resulting in characteristic surface of the iris meets the fibrous tunic is the iridescence (DYCE et al., 1997). iridocorneal angle, where the aqueous humor produced by the ciliary body is reabsorbed in the venous CILIARY BODY circulation (FRANDSON et al., 2005). The anterior surface of the iris is lined with

The ciliary body is a circumferential thickening simple squamous epithelium, the continuation of the of the vascular tunic that gives rise to many fine corneal endothelium. A slightly vascularized suspensory ligaments that support the lens. It is connective tissue follows with few fibers, large involved with the accommodation of the lens and is quantities of pigment cells and fibroblasts, followed by responsible for the production of aqueous humor in the a layer rich in blood vessels embedded in loose anterior segment of the eye bulb (FRANDSON et al., connective tissue. The posterior surface is covered by 2005). the same epithelial layer overlying the ciliary body, According to Gelatt (2003), the ciliary body is and in this region the layer is richer in melanin macroscopically divided into: pleated anterior part, iris (JUNQUEIRA & CARNEIRO, 2004). crown and posterior flat part. The outer most anterior part of the ciliary body forms the ciliary slit. It is AQUEOUS HUMOR shaped like a ring with ridges radiating towards the lens, in the center, and before the ring continues with The aqueous humor is a transparent liquid the iris (DYCE et al., 1997). found in the anterior and posterior chambers of the eye. The adult ciliary body is constituted by two The production and absorption rates are high enough to layers of epithelium. The non-pigmented inner layer is replenish the total volume of the chambers, several closest to the vitreous while the outer layer is times a day (CUNNINGHAM, 2004). In healthy pigmented. The pigmented layer is continuous to the animals, the production rate is proportional to the retinal pigment epithelium (SLATTER, 2005). drainage rate while keeping a constant pressure (DYCE The ciliary body coating consists of a bicellular et al., 1997). epithelial layer whose basic component is the The secretion of aqueous humor is performed connective tissue within which lies the . by the ciliary process of the ciliary body. The material This muscle is constituted by three smooth muscle flows from the posterior to the anterior chamber

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through the pupil and is absorbed by the venous system pupillary edge. Only the posterior two thirds of the in the angle between the cornea and the iris retina can be affected by the light entering the pupil (CUNNINGHAM, 2004). The aqueous humor is (DYCE et al., 1997). drained into the venous system by the canal of Retinal blood vessels are visible through an Schlemm, the irregular edge canal in the corneal ophthalmoscope on the surface of the retina. They form stroma. This is possible due to the presence of a system a network of arteries and veins that enter the retina of labyrinth spaces, the Fontana spaces, which extend through the optic disk and supply increased retinal from the cornea endothelium to the canal of Schlemm nutrition while the remainder is supplied by choroidal (JUNQUEIRA & CARNEIRO, 2004). vessels (CUNNINGHAM, 2004).

VITREOUS OPTICAL DISC

The vitreous humor is an acellular gel that fills The axons of the ganglion cells of the retina the , the space between the lens and leave the eye via the so-called optical disk to form the the (FRANDSON et al., 2005), and consists optic nerve, which is usually round, but may also be primarily of water, hyaluronic acid and collagen fibrils. elliptical (BICAS, 1997). There are no photoreceptors Transparent, it keeps the debris from embryonic in the optical disk, which makes this region completely vitreous, hyaloid artery, and squamous cells of the blind (RAMOS, 2006). anterior uvea and retina (BICAS, 1997). Arterioles and venules emerge from the optic Unlike the aqueous humor, the vitreous humor disc, spreading in several specific patterns for each is not continuously replenished, having a constant species to nourish and drain the retina. The arterioles volume (DYCE et al. 1997). It forms one of the are branches of central retinal artery (DYCE et al. refractive media of the eye and provides the necessary 1997). pressure to properly position the retina against the pigmented epithelium (Gelatt, 2003). OPTIC NERVE According to Dyce et al. (1997), the lens in the embryo is nourished by the hyaloid artery, a branch of The innervation of the eye and its accessory the central retinal artery which passes through the structures originate in six or more cranial nerves. The . The artery usually degenerates after majority of them enter the orbital cone, but some reach birth and the lens is then nourished by diffusion. directly the accessory structures (DYCE et al., 1997). The optic nerve is a cranial nerve formed from RETINA the optical disc; the two optic nerves have more axons than all the dorsal roots of the spinal cord together

The retina is the most complex structure of the (CUNNINGHAM, 2004). Composed primarily of eye; it converts light energy into chemical energy to axons of retinal ganglion cells, the optic nerve connects generate the electrical signal that is carried to the brain. the retina to the visual and non-visual centers in the It is the most metabolically active tissue in the body brain (BROOKS, 2005). (per unit weight), as indicated by oxygen consumption, The optic nerve begins as a single layer of cells and choroidal circulation supplies the entire peripheral with a central lumen. The nerve fibers, which originate retina and the retinal layers in the vicinity of the optic from the ganglion cells in the inner neuroblastic layer, disc (BROOKS, 2005). grow toward the optic stalk, forming the nerve fiber According to Brooks (2005), the retina is layer of the retina and finally the optic nerve classically described as a structure of ten layers, which (SLATTER, 2005). are from outer (sclera) to inner (vitreous): retinal According to Dyce et al. (1997), the optic nerve pigment epithelium (not pigmented in the tapetal enters the orbit through the optical foramen towards the region); rod and cone photoreceptors; external limiting photoreceptor cells of the retina. It is very loose in membrane; (contains rods and order to enable eye movements, and coated by cones cell bodies); (synapse layer meninges, acquired during its development as the stem between photoreceptor axons and dendrites of bipolar of the (Figure 2). and horizontal cells); (contains the üller LENS cell bodies of bipolar, horizontal, ganglion and M cells); (synapse layer between the inner nuclear layer and ganglion cells); ganglion cell The lens is a transparent, biconvex, layer (a single layer of cells whose axons form the proteinaceous disk suspended between the posterior optic nerve); nervous fiber layer (contains the axons of chamber and the vitreous. It is surrounded by an elastic ganglion cells); and, the internal limiting membrane capsule which serves as the attachment site by the (separates the retina from the vitreous). suspensory ligaments at the lens equator (FRANDSON The inner or nervous layer of the eye contains et al., 2005). photosensitive receptor cells and is known as the The lens is composed of three parts, the lens retina. It is an extension of the brain, which remains fibers, long thin prismatic elements; the lens capsule, connected through the optic nerve. The retina begins an acellular coating that is homogeneous, hyaline and where the optic nerve penetrates the choroid, shaped as thicker in the anterior lens epithelium; and, the a concave cup, covers the choroid and ends at the subcapsular epithelium, a single layer of cuboidal

46 epithelial cells present only at the anterior portion of frontonasal process and the lower from the the lens (JUNQUEIRA & CARNEIRO, 2004). maxillary process. The lens, unlike its adjacent liquid is a solid Getty (1986) states that the innervation of the structure, but elastic enough to change its shape. It is upper lid is made by the infratrochlear nerve that biconvex and has anterior and posterior pole, equator distributes in the skin of the medial portion of the and a central axis which coincides with the optical axis upper eyelid and the adjacent area, runs dorsorostral of the eye (DYCE et al., 1997). The lens depends on and then, ventral to the oblique dorsal muscle (Figure the aqueous humor for its nutrition and excretion, as 3). the fiber and lens epithelial cells depend, almost The histological constitution of the eyelids, exclusively, on the glucose metabolism which is from outside to inside, starts with the skin formed by contained in it for energy production (SHIMAMURA, keratinized stratified squamous epithelium and dermis 2008). of the connective tissue. This dense connective tissue According to Junqueira & Carneiro (2004), the forms the palpebral plate where the eyelid sebaceous lens is held in position by a system of radially oriented glands are found while the mucosa consists of a fibers, called the ciliary zonule. The zonular fibers stratified prismatic epithelium (JUNQUEIRA & insert on one side into the lens capsule and on the CARNEIRO, 2004). other, into the ciliary body. The lens capsule is elastic and under permanent tension and would assume a more THIRD EYELID spherical shape without the opposition exerted on the periphery (DYCE et al., 1997). Gelatt (2003) reported that the nictitating The sight of objects at small and large distances membrane or third eyelid is a thin layer of tissue in the is related to the shape of the lens. To view objects more medial of domestic animals. It is a mobile than six meters away, the suspensory ligaments exert protection structure, located between the cornea and traction on the equatorial region of the lens, due to a lower lid, in the nasal portion of the lower conjunctival relaxation of the ciliary muscles of the ciliary body, sac. The muscles controlling the third eyelid are causing anteroposterior flattening of the lens which vestigial in domestic animals while the membrane reduces the refraction of rays. For objects closer to the moves passively on the eye when it is retracted by the eye, the lens assumes a more spherical shape due to eye retractor muscle (SLATTER, 2005). decreasing traction on the suspensory ligaments caused The third eyelid gland is located on the inner by contraction of the ciliary muscles of the ciliary body surface of the third eyelid. The is flat (CUNNINGHAM, 2004). and tubuloalveolar and located on alveolar part of the eyeball (SLATTER, 2005). EYELIDS CONJUNCTIVA

According to Frandson et al. (2005), the eyelids are formed by two moving folds of skin, one upper and The conjunctiva is a mobile anatomical one lower, and the gap between them is the palpebral structure, semi-transparent and likely to be moist and fissure. They offer mechanical protection and have shiny. It lines the inner surface of the eyelids, internal specialized structures to protect the eyeball as cilia and and external of the third eyelid and the anterior portion mucus-producing glands (LUCCI et al., 2006; of the globe adjacent to the limbus. The space covered NARIKAWA et al., 2007). by the conjunctiva is called the conjunctival sac The eyelids emerge from the orbital bone edges (SANTOS, 2011). Rito (2009) stated that it is and, like curtains, are intermittently directed over the abundantly supplied by blood and it is the only exposed portion of the eye and remain closed during lymphatic drainage system of the eye. sleep, when sight is not required (DYCE et al., 1997). The conjunctival epithelium has cubic The cilia are present on the outer surface of the stratification and its lamina propria consists of loose upper eyelid edge in dogs, horses, cattle, pigs and connective tissue (JUNQUEIRA & CARNEIRO, sheep. Few cilia are present in the lower eyelid of 2004). horses, cattle and sheep while cats have no cilia EXTRAOCULAR MUSCLES (SLATTER, 2005). The eyelid movement is caused by its own muscles while the muscle responsible for eyelid According to Getty (1986), the eye has several closure is innervated by the (VIETH et al., muscles external to the orbit, highlighting among them: 1995). The closure of the physically the orbicularis oculi, the sphincter of the eyelids; the protects the cornea from trauma and is also involved in medial levator of the eye angle that runs from the the production, distribution and drainage of the corneal nasofrontal fascia, rostromedially to join the orbicularis tear film. In addition, the eyelids help remove foreign oculi muscle towards the medial corner and lifts the bodies and oxygenate the cornea (SLATTER, 2005). medial portion of the upper eyelid; the lateral retractor According to Rito (2009), the surface ectoderm muscle of the eye angle, which arises from the gives rise to eyelid structures like skin, cilia and temporal fascia caudal to the lateral corner of the eye conjunctival epithelium. The mesenchyme of the and runs rostrally as a narrow band that dips into the neural crest gives rise to deeper structures such as orbicularis oculi muscle; and, the malar muscle, which dermis and . The upper eyelid develops from the 47

ARS VETERINARIA, Jaboticabal, SP, v.29, n.1, 042-051, 2013. ISSN 2175-0106

Figure 2 - Optic nerve of a dog eye (arrow). Source: SA, 2012. Laboratory of Experimental Ophthalmology - UFRPE.

Figure 3 - A: zygomatic branch (1), palpebral branches of the zygomatic branch (*), lacrimal nerve (2) and nerve infratrochlear (3) of a dog. B: infratrochlear nerve (arrow) of a dog. Source: SA, 2012. Laboratory of Experimental Ophthalmology - UFRPE.

Figure 4 - The internal muscles of the eye orbits of a dog, periorbital tissue (1), dorsal rectus muscle (2), (3) and ventral rectus muscle (4). Source: SA, 2012. Laboratory of Experimental Ophthalmology - UFRPE.

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Figure 5 - Lacrimal gland (*) and the lacrimal nerve (arrow) of a dog. Source: SA, 2012. Laboratory of Experimental Ophthalmology - UFRPE.

consists of vertical fibers that open the lower eyelid the orbit ligament and joins the zygomatic and frontal and acts to depress it. nerve in the formation of the auricular plexus; The muscles that move the eye are located however, in some cases it may arise from the maxillary behind the eyeball. All, except one, originate near the nerve (Figure 5) (GETTY, 1986). optic foramen, at the apex of the orbital cone (DYCE et The tear is drained through the lacrimal points al., 1997). and canalicuri and the , which consist The extraocular muscles are formed by seven of mucosal stratified epithelium containing some striated muscles that give movement to the eyeball, the mucous glands and hair cells that contribute to tear bulb retractor muscle; oblique dorsal and ventral drainage (ROSA, 2011). muscles; and, the rectus dorsal, ventral, medial and The tear film is a trilaminate bed essential for lateral muscles (Figure 4) (FRANDSON et al., 2005). maintaining the integrity of the ocular surface According to Dyce et al. (1997), all four recti (FONSECA, 2011). It is an essential layer of muscles are inserted anterior to the equator, by means protection, whose disappearance leads to serious of wide but very thin tendons. The oblique muscles changes in the conjunctiva (SLATTER, 2005). attach to the eyeball near the equator and, due to According to Rite (2009), it is secreted by the lacrimal, contraction, tend to rotate the eyeball around the visual tarsal and third eyelid glands, as well as the axis. The retractor bulb emerges from the vicinity of conjunctival goblet cells. the optic foramen, and is inserted in the eyeball Dyce et al. (1997) described the three layers of posterior to the equator. the tear film. The outer lipid layer originates from the secretion of the tarsal glands and helps spread , LACRIMAL APPARATUS slowing the dissolution of the film. The middle

aqueous layer derived from the lacrimal glands, The lacrimal glands consist of serous cells that dampens and nourishes the cornea. Last, the innermost contain in the apex, secretory granules that stain mucus layer produced by the goblet cells of the weakly while the secretory portion is surrounded by conjunctiva keeps the tear film very close to the myoepithelial cells. These glands produce a saline cornea. secretion with the same sodium chloride concentration of the blood, which is poor in protein and contains VISUAL FIELD lysozyme, an enzyme that digests the capsule of certain bacteria (JUNQUEIRA & CARNEIRO, 2004). The eyes of domestic mammals protrude more The apparatus comprises a series of serous and from the face compared to primates, including man. seromucous lacrimal glands and systems of ducts that The eye position on the head is related to the drain their secretion from the conjunctival sac environment, habits and feeding methods. In general, (FRANDSON et al., 2005). predators have eyes at the front while preys have eyes The lacrimal gland is flat, located between the placed laterally for greater peripheral or side vision eyeball and the dorsolateral wall of the orbit (DYCE et (DYCE et al., 1997). al., 1997). According to Cunningham (2004), it The portion of the environment from which the produces tears in response to parasympathetic nerve light enters the eyes and stimulates the retina is the stimulation. visual field (FRANDSON et al., 2005). According to The lacrimal nerve arises from the ophthalmic Slatter (2005), in prey animals, the eyes are positioned nerve and runs rostrodorsal along the dorsal rectus with divergent visual axes and the total visual field °. The binocular field, the area where muscle to finish in the lacrimal gland. It emerges from approaches 360

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the light from a single object falls on both , is °. Anatomia dos animais domésticos relatively small, around 65 GETTY, R. . 5. ed. Predators on the other hand, have visual axis Rio de Janeiro: Guanabara Koogan, 1986. p. 2000. closer to parallel, more frontal eyes, larger binocular °, and, subsequently, a wider blind zone. The Histologia field, 85 JUNQUEIRA, L. C.; CARNEIRO, J. Básica wide binocular field offers greater accuracy in depth . 10. ed. Rio de Janeiro: Guanabara Koogan, perception and increased coordination with body 2004. p. 488. movements (SLATTER, 2005). ção: análise de 126 LOVATO, F. V. et al. Eviscera ástica ocular do procedimentos realizados no setor de pl CONCLUSION Hospital Banco de Olhos de Porto Alegre entre 1988 e Revista Brasileira de Oftalmologia 2002. . v. 64, n. 4, The ocular system of domestic animals is p. 257-261, 2005. composed by anatomical components which contribute âneo to the visual function. LUCCI, L. M. D. et al. Retalho miocut ção do ectrópio cicatricial. bipediculado para corre Arquivos Brasileiros de Oftalmologia . v. 69, n. 2, p. ACKNOWLEDGMENTS 187-192, 2006.

á ções dos cílios em Thanks are due to Professor Dr. Fabrizio de S NARIKAWA, S. et al. Altera álmica. Arquivos Bezerra for guiding the preparation of the anatomical portadores de cavidade anoft Brasileiros de Oftalmologia specimens and for supplying the used images. . v. 70, n. 1, p. 51-54, 2007.

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