Human_Histology

Suprachoroid Lamina

The suprachoroid lamina is also called the lamina fusca. It is non­vascular. It is made up of delicate connective tissue containing collagen, elastic fibers, and branching cells containing pigment. A plexus of nerve fibers is pre­ sent. Some neurons may be seen in the plexus.

Basal Lamina

With the light microscope the basal lamina (or membrane of Bruch) appears to be a homogeneous layer. However, with the EM the membrane is seen to have a middle layer of elastic fibers, on either side of which there is a layer of delicate collagen fibers. The basal lamina is said to provide a smooth surface on which pigment cells and receptors of the retina can be arranged in precise orientation.

Ciliary Body

The ciliary body represents an anterior continuation of the choroid. It is a ring­like structure continuous with the periphery of the iris. It is connected to the lens by the suspensory ligament.

Theciliarybodyismadeupofvasculartissue,connective tissue and muscle. The muscle component constitutes the ciliaris muscle. The ciliaris muscle is responsible for producing alterations in the convexity of the lens (through the suspensory ligament) enabling the eye to see objects at varying distances from it. In other words the ciliaris is responsible for accommodation.

Theinnersurfaceoftheciliarybodyislinedbyadouble layered epithelium. The outer cell layer is pigmented, whereas the inner cell layer (facing the posterior chamber) is non­pigmented. The cells of the inner layer secrete

aqueous humor. The anterior part of the inner surface of the ciliary body has short processes toward the lens, known as ciliary processes.

Iris

The iris is the most anterior part of the vascular coat of the eyeball. It forms a diaphragm placed immediately in front of the lens. At its periphery it is continuous with the ciliary body. In its center, there is an aperture the pupil. The pupil regulates the amount of light passing into the eye.

The iris is composed of a stroma of connective tissue containing numerous pigment cells, and in which are embedded blood vessels and smooth muscle. Some smooth muscle fibers are arranged circularly around the pupil and constrict it. They form the sphincter pupillae. Other fibers run radially and form the dilator pupillae. The posterior surface of the iris is lined by a double layer of epithelium continuous with that over the ciliary body. This epithelium represents a forward continuation of the retina. The cells of this epithelium are deeply pigmented.

RETINA

This is the inner coat of eyeball and lines its posterior ¾ surface. The retina contains photo­receptors (rods and cones) which are essential for vision. Retina has a specialized area where vision is most acute, called as fovea centralis or macula (Fig. 21.5). This area contains only cones which are essentially bare (the over­lying layers are pushed to the side). The retina also has a “blind spot”, the optic disc, where the optic nerve leaves the eye and there are no photoreceptor cells.

Embryological Considerations

To understand the structure of the retina brief reference to its development is necessary (Figs. 21.4Aand B). The retina develops as an outgrowth from the brain (diencephalon). The proximal part of the diverticulum remains narrow and is called the optic stalk. It later becomes the optic nerve. The distal part of the diverticulum forms a rounded hollow structure called the optic vesicle.This vesicle is invaginated by the developing lens (and other surrounding tissues) so that it gets converted into a two layered optic cup. At first, each layer of the cup is made up of a single layer of cells. The outer layer persists as a single layered epithelium that becomes pigmented. It forms the pigment cell layer of the retina. Over the greater part of the optic cup the cells of the inner layer multiply to form several layers of cells that become the nervous layer of the retina. In the anterior part, both layers of the optic cup remain single layered.

Fig. 21.5: Some features of the retina as seen through an ophthalmoscope (Schematic representation). Note the arteries emerging through the optic disc. Veins are omitted for the sake of clarity

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These two layer sline (a) the inner surface of the ciliary body forming the ciliary part of the retina; and (b) the posterior surface of the iris forming the iridial part of the retina.

Opposite the posterior pole of the eyeball the retina shows a central region about 6 mm in diameter. This region is responsible for sharp vision. In the center of this region an area about 2 mm in diameter has a yellow color and is called the macula lutea (Fig. 21.5). In the center of the macula lutea there is a small depression that is called the fovea centralis. The floor of the fovea centralis is often called the foveola. This is the area of clearest vision.

The optic nerve is attached to the eyeball a short distance medial to the posterior pole. The nerve fibers arising from the retina converge to this region, where they pass through the lamina cribrosa. When viewed from the inside of the eyeball this area of the retina is seen as a circular area called the optic disc.

Basic Structure of the Retina

When we examine sections through the retina (stained by hematoxylin and eosin, Fig. 21.6) a number of layers can be distinguished. The significance of the layers becomes apparent, however, only if we study the retina using special methods. A highly schematic presentation of the layers of the retina, and of the cells present in them is shown in Figure 21.6. The retina can be said to have an external surface that is in contact with the choroid, and an internal surface that is in contact with the vitreous. Beginning from the external surface the following layers can be made out (Plate 21.2).

Pigment Cell Layer: It is the outermost layer of retina which is separated from choroid by Bruch’s membrane. This consists of a single layer of low cuboidal cells containing melanin pigment. Processes from pigment cells extend into the next layer. This layer performs the following functions:

It absorbs and prevents reflection of light that has passed through the neural layers of the retina.

Thepigmentcellsphagocytosetheshedmembranous discs of the outer segment of rods and cones.

These cells also produce melanin.

Layer of Rods and Cones: The rods are processes of rod cells, and cones are processes of cone cells. The peri­ pheral process is rod shaped in the case of rod cells, and cone shaped in the case of cone cells.

External Nuclear Layer: The external nuclear layer contains the cell bodies and nuclei of rod cells and of cone cells.

These cells are photoreceptors that convert the stim­ ulus of light into nervous impulses. Each rod cell or cone cell can be regarded as a modified neuron. It con­ sists of a cell body, a peripheral (or external) process, and a central (or internal) process. The peripheral pro­ cesses lie in the layer of rods and cones described above. The nuclei of these cells are arranged in several layers in the form of external nuclear layer. This layer is darkly stained.

Between second and third layer there is presence of a pink linear marking called as outer limiting membrane or lamina. This results because of zonula adherens of the glial cells (Muller cells) with the cell bodies of photo­ receptor cells. The Muller cells are supporting cells of retina. They have long slender body that is radially orien­ ted in retina. The central process of each rod cell or cone

cell is an axon. It extends into the external plexiform lay­ er where it synapses with dendrites of bipolar neurons.

External Plexiform Layer: The external plexiform layer (or outer synaptic zone) consists only of nerve fibers that form a plexus. The axons of rods and cones synapse here with dendrites of bipolar neurons and horizontal cells. This layer stains lightly.

Internal Nuclear Layer: The internal nuclear layer contains the cell bodies and nuclei of three types of neurons.

The bipolar cells give off dendrites that enter the

external plexiform layer to synapse with the axons of rod and cone cells; and axons that enter the internal plexiform layer where they synapse with dendrites of ganglion cells. The bipolar cells are oriented per­ pendicular to the layers of retina.

The horizontal cells give off processes that run para­ llel to the retinal surface. These processes enter the outer plexiform layer and synapse with rods, cones, and dendrites of bipolar cells. The horizontal cells are oriented parallel to the layers of retina.

The amacrine cells also lie horizontally in the retina. Their processes enter the inner plexiform layer where they synapse with axons of bipolar cells, and with dendrite of ganglion cells.

Note: Apart from bipolar, horizontal and, amacrine neurons, the internal nuclear layer also contains the nuclei of retinal glio­ cytes or cells of Muller (Fig. 21.6). These cells give off numerous

protoplasmic processes that extend through almost the whole thickness of the retina. Externally, they extend to the junction of the layer of rods and cones with the external nuclear layer. Here the processes of adjoining gliocytes meet to form a thin external limiting membrane. Internally, the gliocytes extend to the inter­ nal surface of the retina where they form an internal limiting membrane. This membrane separates the retina from the vitreous. The retinal gliocytes are neuroglial in nature. they support the neurons of the retina and may ensheath them. They probably have a nutritive function as well.

Internal Plexiform Layer: The internal plexiform layer

(or inner synaptic zone) consists of synapsing nerve fibers. The axons of bipolar cells synapse with dendrites of ganglion cells; and both these processes synapse with processes of amacrine cells. The internal plexiform layer also contains some horizontally placed internal plexi­ form cells; and also a few ganglion cells.

Layer of Ganglion Cells: The layer of ganglion cells contains the cell bodies of ganglion cells. We have seen that dendrites of these cells enter the internal plexiform layer to synapse with processes of bipolar cells and of amacrine cells. Each ganglion cell gives off an axon that forms a fiber of the optic nerve.

Layer of Optic Nerve Fibers: The layer of optic nerve fibers is made up of axons of ganglion cells. The fibers converge on the optic disc where they pass through foramina of the lamina cribrosa to enter the optic nerve.

Added Information

Horizontal neurons are of two types, rod horizontals and cone horizontals, depending on whether they syn­ apse predominantly with rods or cones. Each horizontal cell gives off one long process, and a number of short processes (7 in case of rod horizontal cells, and 10 in case of cone horizontal cells). The short processes are specific for the type of cell: those of rod horizontals synapse with a number of rod spherules, and those of cone horizontals synapse with cone pedicles. The long processes synapse with both rods and cones (which are situated some distance away from the cell body of the horizontal neuron). The long and short processes of hori­ zontal cells cannot be distinguished as dendrites or axons, and each process probably conducts in both directions.

The term amacrine is applied to neurons that have no true axon. Like the processes of horizontal cells those of amacrine neurons also conduct impulses in both direc­ tions. Each cell gives off one or two thick processes that divide further into a number of branches. The amacrine cells are believed to play a very important role in the interaction between adjacent areas of the retina resulting in production of sharp images. They are also involved in the analysis of motion in the field of vision.

Chapter 21 Special Senses: Eye 271

Appearance of the Retina in Sections

Having considered the structures comprising the various layers of the retina it is now possible to understand the appearance of the retina as seen in sections stained by hematoxylin and eosin (Plate 21.2). The inner and outer nuclear layers can be made out even at low magnification. The outer nuclear layer is thicker, and the nuclei in it more densely packed than in the inner nuclear layer. We have seen that this (outer nuclear) layer contains the nuclei of rods and cones. The cone nuclei are oval and lie in a single row adjoining the layer of rods and cones. The remaining nuclei are those of rods.

The nuclei in the inner nuclear layer belong (as explai­ ned above) to bipolar cells, horizontal cells, amacrine cells, and gliocytes.

The layer of ganglion cells is (at most places) made up of a single row of cells of varying size. The cell outlines are indistinct, but the nuclei can be made out. They are of various sizes. On the whole they are larger and stain more lightly than nuclei in the inner and outer nuclear layers.

The layer of pigment cells resembles a low cuboidal epithelium. All the nuclei in this layer are of similar size, and lie in a row.

The remaining layers (layers of rods and cones, inner and outer plexiform layers, and the layer of optic nerve fibers) are seen as light staining areas in which no detail can be made out. The layer of rods and cones may show vertical striations.

We will now consider the individual cells of the retina in greater detail.

Blood-retina Barrier

The blood vessels that ramify in the retina do not supply the rods and cones. These are supplied by diffusion from choroidal vessels. The endothelial cells of capillaries in the retina are united by tight junctions to prevent diffusion of substances into the rods and cones. This is referred to as the blood­retina barrier.

Pigment Cells

Pigment cells appear to be rectangular in vertical section, their width being greater than their height (Fig. 21.7). In surface view they are hexagonal. The nucleus is basal in position. The pigment in the cytoplasm is melanin. With the EM it can be seen that the surface of the cell shows large microvilli that contain pigment. These microvilli project into the intervals between the processes of rods and cones. Each pigment cell is related to about a dozen rods and cones. The plasma membrane at the base of the cell shows numerous infoldings.

Plate 21.2: Eye Ball

Eye ball (Schematic representation)

Fig. 21.7: Some features of a pigment cell of the retina (Schematic representation)

The wall of the eye ball is made up of several layers as follows:

1.Sclera, made up of collagen fibers

2.Choroid,containingbloodvesselsandpigmentcells.Theremaining layers are subdivisions of the retina

3.Pigment cell layer

4.Layer of rods and cones

5.Outer nuclear layer

6.Outer plexiform layer

7.Inner nuclear layer

8.Inner plexiform layer

9.Layer of ganglion cells

10. Layer of optic nerve fibers.

The appearance is not likely to be confused with any other tissue.

The functions attributed to pigment cells include the following.

The absorption of excessive light and avoidance of back reflection.

They may play a role in regular spacing of rods and cones and may provide mechanical support to them.

They have a phagocytic role. They “eat up” the ends of rods and cones (which are constantly growing: see below).

Rods and Cones (Fig. 21.8)

There are about seven million cones in each retina. The rods are far more numerous. They number more than 100 million. The cones respond best to bright light (photopic vision). They are responsible for sharp vision and for the discrimination of color. Rods can respond to poor light (scotopic vision) and specially to movement across the field of vision.

Each rod is about 50 µm in length and about 2 µm thick. Cones are about 40 µm in length and 3–5 µm thick.

Fig. 21.8: The main parts of rods and cones

(Schematic representation)

Ultrastructure of Rod and Cone Cells

The ultrastructure of rod cells and of cone cells is similar and is, therefore, considered together. Each rod or cone cell consists of a cell body containing the nucleus, and of external and internal processes, an inner fiber and spherule (Fig. 21.9). The parts of cone cells are almost same except the terminal part which is called pedicle instead of spherule.

The cell body (lying in the external nuclear layer) gives off two “fibers”, inner and outer. The outer fiber passes out­ wards up to the external limiting membrane and becomes continuous with the rod process, or the cone process. The process itself can be divided into an inner segment, and an outer segment. The outer segment is the real photo­ receptor element. It contains a large number of memb­ ranous discs stacked on one another. It is believed that the discs are produced by the cilium (see below) and grad­ ually move toward the tip of the outer segment. Here old discs are phagocytosed by pigment cells.

The outer segments of rods and cones contain photo­ sensitive pigments that are concerned with the conver­ sion of light into nerve impulses. The pigments are belie­ ved to be bound to the membranes of the sacs of the outer segments. The pigment in the rods is rhodopsin, and that in the cones is iodopsin. Cones are believed to be of three types, red sensitive, green sensitive, and blue sensitive. Iodopsin has, therefore, to exist in three forms, one for each of these colors. However, the three types of cones cannot be distinguished from one another on the basis of their ultrastructure.

Fig. 21.9: Structure of a rod cell as seen by electron microscope

(Schematic representation)

The inner segment of the rod or cone process is wider than the outer segment. It contains a large number of mito­ chondria that are concentrated in a region that is called the ellipsoid.

At the junction of the inner and outer segments of the rod or cone process there is an indentation of the plasma membrane on one side, so that the connection becomes very narrow. This narrow part contains a fibrillar cilium in which the microfibrils are orientated as in cilia elsewhere. This cilium is believed to give rise to the flattened discs of the outer segment.

Fig. 21.10: Rod spherule synapsing with terminals of rod bipolar cells and horizontal cells (Schematic representation)

We have seen that the part of the rod cell between the cell body and the external limiting membrane is the outer fiber. The length of the outer fiber varies from rod to rod, being greatest in those rods that have cell bodies placed “lower down” in the external nuclear layer. The outer fiber is absent in cones, the inner segment of the cone process being separated from the cone cell body only by a slight constriction.

The cell bodies of rod cells and of cone cells show no particular peculiarities of ultrastructure.

The inner fibers of rod and cone cells resemble axons. At its termination each rod axon expands into a spherical structure called the rod spherule, while cone axons end in expanded terminals called cone pedicles (Figs. 21.10 and 21.11). The rod spherules and cone pedicles form complex synaptic junctions with the dendrites of bipolar neurons, and with processes of horizontal cells. Each rod spherule synapses with processes of two bipolar neurons, and with processes of horizontal neurons.

Each cone pedicle has numerous synapses with pro­ cesses from one or more bipolar cells, and with processes of horizontal cells. In many situations the cone pedicle bears several invaginations that are areas of synaptic con­ tacts. Each such area receives one process from a bipolar dendrite; and two processes, one each from two horizontal neurons. Such groups are referred to as triads. Each cone pedicle has 24 such triads. Apart from triads the cone pedicle bears numerous other synaptic contacts in areas intervening between the triads. These areas synapse with dendrites of diffuse bipolar cells. Some pedicles also esta­ blish synaptic contacts with other cone pedicles.

Fig. 21.11: Cone pedicle showing a number of synaptic areas, each area receiving three terminals (Schematic representation)

Clinical Correlation

Retinal Detachment

Retinal detachment is the separation of the neurosensory retina from the retinal pigment epithelium. It may occur spontaneously in older individuals past 50 years of age or may be secondary to trauma in the region of head and neck. There are 3 pathogenetic mechanisms of retinal detachment:

Pathologic processes in the vitreous or anterior segment

Collection of serous fluid in the sub-retinal space

Accumulation of vitreous under the retina through a hole or a tear in the retina.

Retinitis Pigmentosa

Retinitis pigmentosa is a group of systemic and ocular diseases of unknown etiology, characterized by degeneration of the retinal pigment epithelium. The earliest clinical finding is night blindness due to loss of rods and may progress to total blindness.

Retinoblastoma

This is the most common malignant ocular tumor in children. It may be present at birth or recognized in early childhood before the age of 4 years. About 60% cases of retinoblastoma are sporadic and the remaining 40% are familial. Familial tumors are often multiple and multifocal and transmitted as an autosomal dominant trait by retinoblastoma susceptibility gene (RB) located on chromosome 13. Such individuals have a higher incidence of bilateral tumors and have increased risk of developing second primary tumor, particularly osteogenic sarcoma. Clinically, the child presents with leukokoria, i.e. white pupillary reflex.

LENS

The lens of the eye is a transparent biconvex avascular structure. It is suspended between the iris and the vitreous

Fig. 21.12: Section through part of the lens near its margin (Schematic representation)

by the zonules, which connect the lens with the ciliary body. It is surrounded by an elastic capsule which is a semi­ permeable membrane. The posterior surface of the lens is more curved than the anterior.

The lens consists of three parts:

Lens capsule

Lens epithelium

Lens substance (Fig. 21.12).

Lens Capsule

The lens capsule is a transparent homogeneous and highly elastic collagenous basement membrane. It is made up mainly of type IV collagen and glycoproteins. The capsule is thicker in front than behind. It is secreted by the lens epithelium.

Lens Epithelium

Deep to the capsule the lens is covered on its anterior surface by a lens epithelium. The cells of the epithelium are cuboidal. However, toward the periphery of the lens the cells become progressively longer. Ultimately they are converted into long fibers that form the sub­ stance of the lens.

The cells of epithelium are metabolically active, con­ tain Na+­K+­ATPase and generate adenosine triphosphate (ATP) to meet the energy demand of the lens. The cells show high mitotic activity and form new cells which migrate

Fig. 21.13: Arrangement of fibers within the lens. Note the Y-shaped lines on the front and back of the lens (Schematic representation)

toward the equator. The lens epithelial cells continue to divide and develop into the lens fibers.

Lens Fibers

The lens fibers develop from the lens epithelial cells that continue to divide and get elongated and transformed into lens fibers. They are mainly composed of soluble pro­ teins called crystallins.

The fibers formed earlier lie in the deeper plane (nucleus of the lens), the newer ones occupy a more superficial plane.

When the lens is examined from the front, or from behind, three faint lines are seen radiating from the center to the periphery. In the fetus these lines form a “Y” that is upright on the front of the lens, and inverted at the back (Fig. 21.13). The lines become more complex in the adult. These lines are called sutural lines. They are made up of amorphous material. The ends of lens fibers are attached at these lines. Each lens fiber starts on one surface at such a line, and follows a curved course to reach the opposite surface where it ends by joining another such line.

Clinical Correlation

Cataract

The cataract is the opacification of the normally crystalline lens which leads to gradual painless blurring of vision. The various causes of cataract are: senility, congenital (e.g. Down syndrome, rubella, galactosaemia), traumatic (e.g. penetrating injury, electrical injury), metabolic (e.g. diabetes, hypoparathyroidism), drug associated (e.g. long-term corticosteroid therapy), smoking, and heavy alcohol consumption.

276 Textbook of Human Histology

Fig. 21.14: Eyelid (Schematic representation). 1–core of skeletal muscle; 2–dense connective tissue; 3–skin; 4–hair follicle;

5–Sebaceous gland; 6–Sweat gland; 7–Palpebral conjunctiva;

8–Tarsal plate; 9–Tarsal glands; 10–Eyelash; 11–Ciliary gland; 12–Levator palpebrae superioris; 13–Accessory lacrimal glands

ACCESSORY VISUAL ORGANS

The accessory visual organs include the extraocular muscles and related fascia, the eyebrows, the eyelids, the conjunc­ tiva, and the lacrimal gland. The structure of extraocular muscles corresponds to that of skeletal muscle elsewhere in the body; and the structure of eyebrows is similar to that of hair in other parts of the body. The remaining structures are considered below.

Eyelids

Eyelids are two movable skin folds that protect the eye from injury and keep the cornea clean and moist. The basic structure of an eyelid is shown in Figure 21.14.

Anteriorly, there is a layer of true skin with which a few small hair and sweat glands are associated. The skin is thin.

Deep to the skin there is a layer of delicate connective tissue that normally does not contain fat.

Considerable thickness of the lid is formed by fasciculi of the palpebral part of the orbicularis oculi muscle (skeletal muscle).

The “skeleton” of each eyelid is formed by a mass of fibrous tissue called the tarsus, or tarsal plate.

On the deep surface of the tarsal plate there are a series of vertical grooves in which tarsal glands (or Meib­ omian glands) are lodged. Occasionally, these glands may be embedded within the tarsal plate. Each gland has a duct that opens at the free margin of the lid. The tarsal glands are modified sebaceous glands. They pro­ duce an oily secretion a thin film of which spreads over the lacrimal fluid (in the conjunctival sac) and delays its evaporation.

Modified sweat glands, called ciliary glands (or glands of Moll), are present in the lid near its free edge. Seba­ ceous glands present in relation to eyelashes constitute the glands of Zeis. They open into hair follicles. Acces­ sory lacrimal glands are often present just above the tarsal plate (glands of Wolfring).

The inner surface of the eyelid is lined by the palpebral conjunctiva.

Clinical Correlation

Stye (Hordeolum)

Stye or ‘external hordeolum’ is an acute suppurative inflammation of the sebaceous glands of Zeis, the apocrine glands of Moll and the eyelash follicles.

Chalazion

Chalazion is a very common lesion and is the chronic inflammatory process involving the meibomian glands. It occurs as a result of obstruction to the drainage of secretions. The inflammatory process begins with destruction of meibomian glands and duct and subsequently involves tarsal plate.

Conjunctiva

The conjunctiva is a thin transparent membrane that covers the inner surface of each eyelid (palpebral conjunctiva) and the anterior part of the sclera (ocular conjunctiva). At the free margin of the eyelid the palpebral conjunctiva becomes continuous with skin; and at the margin of the cornea the ocular conjunctiva becomes continuous with the anterior epithelium of the cornea. When the eyelids are closed the conjunctiva forms a closed conjunctival sac. The line along which palpebral conjunctiva is reflected onto the eyeball is called the conjunctival fornix: superior, or inferior.

Conjunctiva consists of an epithelial lining that rests on connective tissue. Over the eyelids this connective tissue

is highly vascular and contains much lymphoid tissue. It is much less vascular over the sclera.

The epithelium lining the palpebral conjunctiva is typi­ cally two layered. There is a superficial layer of columnar cells, and a deeper layer of flattened cells. At the fornix, and over the sclera, the epithelium is three layered there being an additional layer of polygonal cells between the two layers mentioned above. The three layered epithe­ lium changes to stratified squamous at the sclerocorneal junction.

Lacrimal Gland

The lacrimal gland is a compound tubuloalveolar gland and consists of a number of lobes that drain through about twenty ducts. It is a tear­secreting gland. The structure of the lacrimal gland is similar to that of a serous salivary gland (Fig. 21.15).

Sections of the lacrimal gland can be distinguished from those of serous salivary glands because of the following features.

The acini are larger, and have wider lumina.

All cells appear to be of the same type. They are low columnar in shape and stain pink with hematoxylin and eosin.

The profiles of the acini are often irregular or elongated.

The walls of adjacent acini within a lobule may be pres­ sed together, there being very little connective tissue between them. However, the acini of different lobules are widely separated by connective tissue. Myoepithelial cells are present as in salivary glands.

Small ducts of the lacrimal gland are lined by cuboidal

or columnar epithelium. Larger ducts have a two layered columnar epithelium or a pseudostratified columnar epithelium.

Fig. 21.15: Lacrimal gland (Schematic representation). 1–Lumen of acinus; 2–Myoepithelial cell; 3–Duct

Chapter 21 Special Senses: Eye 277

EM studies on the human lacrimal gland reveal that the secretory cells may be of several types, including both mucous and serous cells.

The ducts of the lacrimal gland open into the lateral part of the superior conjunctival fornix. Lacrimal fluid keeps the conjunctiva moist. Accessory lacrimal glands are present near the superior conjunctival fornix (glands of Krause).

Added Information

Density of Rods and Cones in Retina

The density of rods and cones in different parts of the retina is shown schematically in Figure 21.16. Note the following points:

The density of cones is greatest in the fovea (about 1.5 million/mm2). Their density decreases sharply in proceed­ ing to the margin of the central area, but thereafter the density is uniform up to the ora serrata (about 5,000/mm2).

The density of rods is greatest at the margin of the cen­ tral area (about 1.5 million/mm2). It decreases sharply on proceeding toward the margin of the central area. There are no rods in the foveola. The density of rods also decreases in passing toward the ora serrata (where it is about 30,000/mm2).

Bipolar Neurons

Bipolar cells of the retina are of various types. The termino­ logy used for them is confusing as it is based on multiple criteria. The main points to note are as follows:

The primary division is into bipolars that synapse with rods (rod­bipolars), and those that synapse with cones (cone­ bipolars).

Fig. 21.16: Scheme to show the relative number of rods and cones in different parts of the retina. The figures represent number of receptors

per mm2. The diagram is not drawn to scale

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