1. Introduction to Anatomy of the Eye and its Adnexa

Transcription

1 1. Introduction to Anatomy of the Eye and its Adnexa Fig 1: A Cross section of the human eye. Let us imagine we are traveling with a ray of light into the eye. The first structure we will encounter is the cornea (Fig 2).This is a transparent structure which allows light to enter inside the eye. It fits into the surrounding white opaque tissue; the sclera like a watchglass and together they constitute the outer coat of the eye. This outer coat protects the eye from injury and invading micro-organisms (Fig 3). The Cornea is made up of 5 layers: The epithelium, Bowman's layer, the corneal stroma, Descemet's membrane and the endothelium Fig 2: The Cornea illuminated by a slit beam. The stroma of the cornea is made up of collagen fibres called lamellae. The corneal lamellae are arranged in a specific order which helps to achieve corneal transparency; other factors for corneal transparency include corneal avascularity, and the presence of active endothelial pump which makes the cornea dry and not misty. Fig 3: The White sclera seen underneath the transparent conjunctiva. Light passing from the cornea enters a space full of clear fluid known as the anterior chamber (Fig 4), this space is bounded by the cornea anteriorly and by the iris posteriorly. The iris is the part which gives the eye its color.

2 Fig 4: The Anterior Chamber. The iris (Fig 5)is made up of muscles, blood vessels and pigmented cells. It has a central circular opening through which light travels, known as the pupil (Fig 5). The pupil is a circular opening in the iris that allows transmission of light. It controls the amount of light reaching the retina by dilating and constricting. The iris contains two types of smooth muscles: The circular constrictor pupillae and the longitudinal dilator pupillae Fig 5: The iris and Pupil The constrictor pupillae is under parasympathetic control and on contraction the pupil narrows The Dilator pupillae is under sympathetic control and on contraction the pupil dilates The detailed anatomy of the innervation of the pupil will be discussed in chapter 9 (Connection of the eye to the brain).

3 The iris is continuous posteriorly with the ciliary body (fig 6) and the choroid. (Fig 7) These 3 structures are made up of blood vessels and contain pigment and are collectively known as the uveal tract or the middle or vascular coat of the eye. Its main function is to provide nutrition and support for the overlying and underlying structures. Fig 6: A diagram of the ciliary body (pars plana and [pars plicata). Fig 7: The Choroid and its blood vessels. This middle coat of the eye would be the first structure to encounter if we were to peel away the sclera as seen in fig 8. The ciliary body is made up of two parts: The pars plicata and the pars plana. Fig 8: The choroid and its blood vessels with the sclera peeled away. The pars plicata is covered with epithelium that secretes aqueous humour and gives attachment to the suspensory ligament or zonule that suspends the crystalline lens in place. It contains a smooth muscle known as the ciliary muscle whose contraction results in relaxation of the zonule and bulging of the lens, thus changing its curvature and power. This is known as accommodation.

4 After light passes through the pupil it meets a transparent object suspended from the ciliary body known as the crystalline lens (Fig 9). The lens is an important refractive medium of the eye. The space between the crystalline lens and the iris is known as the posterior chamber (Fig 10). The transparent fluid that fills the anterior chamber is known as the aqueous humor and is formed by the ciliary body and secreted into the posterior chamber, behind the iris, it then passes through the pupil to fill the anterior chamber and be drained through the angle (Fig 10) between the cornea and sclera. Aqueous formation, circulation and drainage through the angle, in addition to the structures of the angle will be discussed in more detail in chapter 5 (Aqueous humour, IOP and glaucoma). Fig 9: A diagramatic representation of the crystalline lens. Fig 10: Passage of aqueous humour from the ciliary body in the posterior chamber, through the pupil into the anterior chamber.

5 The Lens: The lens is a biconvex avascular structure present behind the pupil. It contributes about +20 D to the refractive power of the eye. This power can be increased during accommodation, which can be achieved by ciliary muscle contraction resulting in relaxation of the zonule and allowing the elastic capsule to change the shape of the lens in order to become more convex. This allows seeing near objects clearly. The Lens is also a dynamic structure that is continuously growing with human growth. The capsule which surrounds the lens is elastic and has an epithelial layer on the inner surface of its anterior portion from which lens proteins are formed. As it grows, the lens proteins become more aggregated in the center and lose fluid. This hardening or sclerosis leads to a distinction of the lens proteins into the harder central nucleus and the softer peripheral cortex. Due to this sclerosis, the lens becomes less pliable by age and consequently loses its ability to increase its power by changing its shape (Accommodation). Once light has been refracted by the lens it meets the transparent gel like structure that fills the space behind the lens. This gel like structure is known as the vitreous body (Fig 11) and (Fig 12). The vitreous is formed of 99% water in addition to special proteins called glycosaminoglycans. No clear function is known to the vitreous apart from transmitting light to the retina. However, it may have a shock absorbing role due to its gel-like state and it may also act as a reservoir for certain nutrients or chemical mediators.

6 Fig 11: A diagramatic representation of the vitreous body. Fig 12: The vitreous body as seen after peeling the sclera, choroid and retina away. Having traversed the vitreous body, light will reach its final destination by falling upon the light sensitive neurosensory structure known as the retina (Fig 13). The retina is the inner coat of the eye. The retina is a very intricate structure which is closely connected anatomically and embryologically to the brain, and will be discussed in more detail in chapter 8 (The Retina: Functions and diseases). Fig 13: Fundus photograph showing the appearance of a normal retina. The retina performs the essential task of converting light to electrical impulses. These impulses are then transmitted via the optic nerve (Fig 14) to the brain. The optic nerve can be actually viewed as being a tract of the central nervous system rather than a true nerve. The occipital cortex is the part of the brain responsible for vision i.e. giving meaning to those electrical impulses. The route from the retina to that part is called the visual pathway (Fig 15). The detailed anatomy of the optic nerve and visual pathway will be discussed in chapter 9 (Connection of the eye to the brain).

7 Fig 14: The Optic nerve seen passing from the back of the eye into the cranial cavity. Fig 15: The visual pathway. Due to the importance of the retina in the process of vision, all the aforementioned ocular structures can be looked at as serving the purpose of allowing light to reach the retina and to provide the retina with protection and nutrition. Additional protection is provided by the ocular adnexa and orbit. The orbit (Fig 16) is a bony socket in-which the eye is placed. Within the orbit the eye is surrounded by fat,muscles, nerves and vessels,in addition to the lacrimal gland (Fig 17) that secretes the main ( watery) part of the tears.the tears are then drained through the lacrimal drainage system (Fig 17, See details in chapter 3)). Fig 16: The Bony orbit. Fig 17: The lacrimal gland and lacrimal drainage system.

8 Protecting the surface of the eye are the eyelids (Fig 18). The eyelids and the outer surface of the eye are covered with a transparent membrane known as the conjunctiva (Fig 19). It lubricates the ocular surface and contributes to tear formation aswell. The anatomy of the orbit, eyelids, lacrimal gland and lacrimal drainage system will be discussed in further details in chapter 4 (Diseases of the protective structures of the eye). Fig 18: A cross-section in the eyelids. Fig 19: A cross-section in the conjunctiva of the upper and lower lids in red. The conjunctiva is a membrane that lines the inner surface of the eyelids and the outer surface of the globe. It is formed of three parts. The palpebral part (related to the lid), the bulbar part (related to the globe) and the fornix, which is the pouch connecting both these parts. The fornix of the lower lid is the site where eye-drops are instilled. The conjunctiva plays a protective role, in addition to its important contribution to tear formation.

9 The muscles in the orbit known as the Extraocular muscles (Fig 20& 21) are responsible for moving the eyes in different directions in asmooth and coordinated way. The anatomy of the extra-ocular muscles and their function will be discussed in further details in chapter 7 (Ocular motility in health and disease). Fig 20: The extra-ocular muscles in both eyes. Fig 21: The extra-ocular muscles in relation to the orbit.