Internal Anatomy Of The

A transverse section through the middle of the eyeball shows three layers. The outermost layer is the sclera. It is white in color and consist of dense fibrous connective tissue. The sclera covers the entire aspect of the eye except the anterior pigmented portion of the eye, the iris. Covering the iris is a transparent, nonvascular fibrous coat called the cornea.

The middle layer of the eye is the choroid. The choroid is pigmented and contains numerous blood vessels. Anteriorly, the choroid consists of the ciliary body and iris. The ciliary body contains smooth muscle called the ciliary muscle which is necessary for changing the shape of the lens of the eye. The iris is a pigmented structure. The more pigment the iris contains, the darker its color. The black hole in the middle of the iris is the pupil. It is through the pupil that light rays enter the eye. Connecting the lens of the eye to the ciliary body is the suspensory ligament.

The innermost layer of the eyeball is the retina. The retina is made up of neurons such as rods and cones. The axons of some other neurons converge and exit through the posterior aspect of the eye. The point of convergence is called the optic disc or blind spot. There are no photoreceptors in the optic disc and hence light striking on this spot cannot be perceived. The axons from the retina pierce the choroid and sclera and are known as the optic nerve (II).

In the exact center of the retina is a depression called the macula lutea. The center of the macula lutea is called the fovea centralis which has the highest concentration of cones within the eye. There are no rods located within the macula lutea.

The interior of the eyeball is divided into two cavities by the lens. The cavity in front of the lens is called the anterior cavity. It is divided into two chambers. The anterior chamber is in front of the iris and the posterior chamber is behind the iris. The anterior cavity contains a watery fluid called the aqueous humor. Aqueous humor is constantly being produced by the choroid plexuses of the ciliary body, passes into the posterior chamber, and then into the anterior chamber. The aqueous humor is then reabsorbed into small spaces called trabeculae. From the trabeculae, the aqueous humor is drained into a venous sinus called the canal of Schlemm. The canal of Schlemm is located at the junction of the sclera and cornea and completely encircles the eye at this point.

The cavity behind the lens of the eye is called the posterior cavity. It contains a jellylike substance called the vitreous body. The vitreous body helps the eyeball to maintain its shape and helps to hold the retina in place. Once the eyeball has completed its development, no additional vitreous body is produced.

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Figure 24.2 Internal Anatomy of the Eyeball

Anterior cavity Anterior chamber Canal of Schlemm Choroid Ciliary body Conjunctiva Cornea Iris Lateral rectus Lens Macula lutea Medial rectus Optic disc (Blind spot) Optic nerve II Posterior cavity Posterior chamber Pupil Retina Sclera Suspensory ligament

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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