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Ciliary muscle fibers relaxed

Suspensory ligaments taut

Lens thin

Ciliary muscle fibers contracted

Suspensory ligaments relaxed

Lens thick

(a) The lens thins as the ciliary muscle fibers relax. (b) The lens thickens as the ciliary muscle fibers contract.

U Describe the outer and middle tunics of the eye.

^9 What factors contribute to the transparency of the cornea?

^9 How does the shape of the lens change during accommodation?

The iris is a thin diaphragm mostly composed of connective tissue and smooth muscle fibers. Seen from the outside, it is the colored portion of the eye. The iris extends forward from the periphery of the ciliary body and lies between the cornea and the lens. It divides the space separating these parts, which is called the anterior cavity, into an anterior chamber (between the cornea and the iris) and a posterior chamber (between the iris and the vitreous humor, occupied by the lens).

The epithelium on the inner surface of the ciliary body continuously secretes a watery fluid called aqueous humor into the posterior chamber. The fluid circulates from this chamber through the pupil, a circular opening in the center of the iris, and into the anterior chamber (fig. 12.31). Aqueous humor fills the space between the cornea and the lens, providing nutrients and maintaining the shape of the front of the eye. It subsequently leaves the anterior chamber through veins and a special drainage canal, the scleral venous sinus (canal of Schlemm), located in its wall at the junction of the cornea and the sclera.

The smooth muscle fibers of the iris form two groups, a circular set and a radial set. These muscles control the size of the pupil, which is the opening that light passes through as it enters the eye. The circular set of muscle fibers acts as a sphincter, and when it contracts, the pupil gets smaller (constricts), and the intensity of the light entering decreases. When the radial muscle fibers contract, the diameter of the pupil increases (dilates), and the intensity of the light entering increases.

The sizes of the pupils change constantly in response to pupillary reflexes triggered by such factors as light intensity, gaze, accommodation, and variations in emotional state. For example, bright light elicits a reflex, and impulses travel along parasympathetic nerve fibers to the circular muscles of the irises. The pupils constrict in response. Conversely, in dim light, impulses travel on sympathetic nerve fibers to the radial muscles of the irises, and the pupils dilate (fig. 12.32).

The amount and distribution of melanin in the irises and the density of the tissue within the body of the

Aqueous humor Iris

Scleral venous sinus (canal of Schlemm)

Sclera

Ciliary body

Vitreous humor Posterior chamber

Figure 12.31

Sclera

Sclera Venous Sinus

Cornea

Anterior chamber

Ciliary body

Vitreous humor Posterior chamber

Aqueous humor (blue arrows), which is secreted into the posterior chamber, circulates into the anterior chamber and leaves it through the canal of Schlemm (scleral venous sinus).

Cornea

Anterior chamber

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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