Chapter Summary

I. EYE

A. Fibrous Tunic

1. Cornea

The cornea is composed of five layers. From superficial to deep, they are the following:

a.Stratified squamous nonkeratinized epithelium

b.Bowman’s Membrane

The outer, homogeneous layer of the stroma

c. Stroma

A transparent, dense, regular, collagenous connective tissue housing fibroblasts and occasional lymphoid cells, comprising the bulk of the cornea

d.Descemet’s Membrane

A thick, basal lamina

e.Corneal Endothelium

Not a true endothelium, a simple squamous-to- cuboidal epithelium

2. Sclera

The sclera, the white of the eye, is composed of three layers: the outer episcleral tissue housing blood vessels; the middle stroma, composed of dense, regular, collagenous connective tissue; and the suprachoroid lamina, a loose connective tissue housing fibroblasts and melanocytes.

B. Vascular Tunic

The vascular tunic (uvea) is a pigmented, vascular layer housing smooth muscles. It is composed of the choroid membrane, the ciliary body, and the iris.

1. Choroid Membrane

The choroid membrane is composed of four layers. The suprachoroid layer is shared with the sclera and houses fibroblasts and melanocytes. The vascular and choriocapillary layers house larger vessels and capillaries, respectively. The glassy membrane (of Bruch), interposed between the choroid and the retina, is composed of basal lamina, collagen, and elastic fibers.

2. Ciliary Body

The ciliary body is the region of the vascular tunic located between the ora serrata and the iris. The ciliary body is composed of the numerous, radially arranged,

aqueous humor–forming ciliary processes that together compose the ciliary crown from which suspensory ligaments extend to the lens. Three layers of smooth muscle, oriented more or less meridianally, radially, and circularly, function in visual accommodation. The vascular layer and glassy membrane of the choroid continue into the ciliary body. The inner aspect of the ciliary body is covered by the inner nonpigmented and outer pigmented layers of the ciliary epithelium.

3. Iris

The iris, separating the anterior from the posterior chamber, is attached to the ciliary body along its outer circumference. The free edge of the iris forms the pupil of the eye. The iris is composed of three layers: the outer (frequently incomplete) simple squamous epithelial layer, a continuation of the corneal epithelium; the intermediate fibrous layer, composed of the nonvascular anterior stromal and vascular general stromal layers that house numerous melanocytes and fibroblasts; and the posterior pigmented epithelium. The sphincter and dilator muscles of the pupil are composed of myoepithelial cells derived from the pigmented epithelium.

C. Retinal Tunic

The retinal tunic, the deepest of the three layers, consists of the pars iridica, pars ciliaris, and pars optica. The last of these is the only region of the retina that is sensitive to light, extending as far anteriorly as the ora serrata, where it is continuous with the pars ciliaris.

1. Pars Optica

The pars optica is composed of 10 layers.

a. Pigment Epithelium

The pigment epithelium is attached to the choroid membrane

b. Lamina of Rods and Cones

The outer and inner segments of the photoreceptor cells form the first layer; the remainder of these cells constitutes the next three layers

c. External Limiting Membrane

The external limiting membrane is not a true membrane.Itismerelyajunctionalspecializationbetween the photoreceptor cells and processes of Müller (supportive) cells

d. Outer Nuclear Layer

The outer nuclear layer houses the cell bodies (and nuclei) of the photoreceptor cells. At the fovea centralis, only cones are present

e. Outer Plexiform Layer

The outer plexiform layer is the region of synapse formation between the axons of photoreceptor cells and the processes of bipolar and horizontal cells

f. Inner Nuclear Layer

The inner nuclear layer houses the cell bodies of Müller, amacrine (associative), bipolar, and horizontal cells

g. Inner Plexiform Layer

The inner plexiform layer is the region of synapses between dendrites of ganglion cells and axons of bipolar cells. Moreover, processes of Müller and amacrine cells are also present in this layer

h. Ganglion Cell Layer

The ganglion cell layer houses the cell bodies of multipolar neurons, which are the final link in the neuronal chain of the retina, and their axons form the optic nerve. Additionally, neuroglia are also located in this layer

i. Optic Nerve Fiber Layer

The optic nerve fiber layer is composed of the unmyelinated axons of the ganglion cells, which are collected as the optic nerve

j. Inner Limiting Membrane

The inner limiting membrane is composed of the expanded terminal processes of Müller cells

2. Pars Ciliaris and Pars Iridica Retinae

At the pars ciliaris and pars iridica retinae, the retinal layer has been reduced to a thin epithelial layer consisting of a columnar and a pigmented layer lining the ciliary body and iris.

D. Lens

The lens is a biconvex, flexible, transparent disc that focuses the incident rays of light on the retina. It is composed of three layers, an elastic capsule (basement membrane), an anteriorly placed simple cuboidal epithelium, and lens fibers, modified epithelial cells derived from the equator of the lens.

E. Lacrimal Gland

The lacrimal gland is external to the eye, located in the superolateral aspect of the orbit. It is a compound tubuloalveolar gland, producing a lysozyme-rich serous fluid with an alkaline pH.

F. Eyelid

The eyelid is covered by thin skin on its external aspect and by conjunctiva, a mucous membrane, on its inner aspect. A thick, dense, fibrous connective tissue tarsal plate maintains and reinforces the eyelid. Associated with the tarsal plate are the tarsal glands, secreting an oily sebum that is delivered to the margin of the eyelid. Muscles controlling the eyelid are located within its substance. Associated with the eyelashes are sebaceous glands. Ciliary glands are located between eyelashes.

II. EAR

A. External Ear

1. Auricle

The auricle is covered by thin skin and is supported by highly flexible elastic cartilage plate.

2. External Auditory Meatus

The external auditory meatus is a cartilaginous tube lined by skin, containing ceruminous glands and some fine hair. The skin of the external meatus is continuous with the external covering of the tympanic membrane. In the medial aspect of the meatus, the cartilage is replaced by bone.

3. Tympanic Membrane

The tympanic membrane is a thin, taut membrane separating the external from the middle ear. It is lined by stratified squamous keratinized epithelium externally and low cuboidal epithelium internally and possesses a core of collagen fibers disposed in two layers.

B. Middle Ear

The middle ear is composed of the simple cuboidal epithelium–lined tympanic cavity containing the three ossicles (malleus, incus, and stapes). The tympanic cavity communicates with the nasopharynx via the cartilaginous and bony auditory tube. The medial wall of the middle ear communicates with the inner ear via the oval

(vestibular) and round (cochlear) windows.

C. Inner Ear

1. Cochlea

The bony cochlea houses the endolymph-filled cochlear duct that subdivides the perilymph-filled cochlea into the superiorly positioned scala vestibuli and the inferiorly located scala tympani.

a. Cochlear Duct

The cochlear duct houses the spiral organ of Corti that lies on the basilar membrane. The spiral organ of Corti is composed of cells of

478S P E C I A L S E N S E S

Claudius, cells of Böttcher, and cells of Hensen, all of which assist in the formation of the outer tunnel along with the outer hair cells and outer phalangeal cells. The tectorial membrane lies over the outer hair cells as well as the inner hair cells, thus forming the internal spiral tunnel. The region between the inner and outer hair cells is occupied by pillar cells, which assist in the formation of the inner tunnel (of Corti). The stria vascularis constitutes the outer wall of the cochlear duct. Nerve fibers lead to the spiral ganglion (housing pseudounipolar cells) in the modiolus.

b. Membranous Labyrinth

The membranous labyrinth is composed of the utricle, the saccule, and the three semicircular canals.

1. Utricle and Saccule

The utricle and saccule are both filled with endolymph and house maculae. Each macula is composed of simple columnar epithelium composed of two cell types, neuroepithelial hair cells and supporting cells. The free surface of the macula displays the otolithic membrane, housing small particles called otoliths.

2. Semicircular Canals

The three semicircular canals are oriented perpendicular to each other. The ampulla of each canal houses a crista, a structure similar to a macula, composed of neuroepithelial hair cells and supporting cells. A gelatinous cupula is located at the free surface of the crista, but it contains no otoliths.