Chapter 16 - Sense Organs - Study Guide - Testing Your Comprehension - Page 628: 3

The contraction of the ciliary muscle is a unique mechanism in the eye that actually reduces tension on the lens. This process is crucial for adjusting the shape of the lens and, consequently, for changing the eye's focal length to focus on objects at different distances. This phenomenon is known as **accommodation** and is necessary for clear vision at various distances. Here's how the contraction of the ciliary muscle reduces tension on the lens: 1. **Lens Structure:** The lens of the eye is a transparent, flexible, and elastic structure located behind the iris. Its main function is to bend (refract) incoming light rays so that they converge onto the retina, forming a focused image. 2. **Lens Shape and Tension:** The shape of the lens is determined by the tension exerted on it by the suspensory ligaments (zonular fibers) that attach to the lens capsule. When the ciliary muscle is at rest, it exerts tension on the suspensory ligaments, which in turn flatten the lens. This is the default position for the lens when focusing on distant objects. 3. **Contraction of the Ciliary Muscle:** When you shift your focus from a distant object to a near one, the ciliary muscle contracts. This contraction has two important effects: - **Relaxation of the Suspensory Ligaments:** The ciliary muscle, when it contracts, relaxes or loosens the tension on the suspensory ligaments. This relaxation allows the lens to regain its natural, more rounded shape. - **Increase in Lens Curvature:** As the suspensory ligaments relax, the lens becomes more curved or bulged in shape. This increased curvature increases the refractive power of the lens, allowing it to bend light rays more strongly. This is necessary to focus on closer objects. 4. **Accommodation:** The changes in lens shape and tension induced by ciliary muscle contraction and relaxation are collectively known as accommodation. Accommodation allows the eye to adjust its focus from distant objects (flatter lens) to near objects (more rounded lens) by changing the curvature of the lens without the need for external devices like glasses. In summary, the contraction of the ciliary muscle reduces tension on the lens by relaxing the suspensory ligaments. This, in turn, allows the lens to adopt a more rounded shape, increasing its refractive power and enabling the eye to focus on objects at closer distances. This mechanism is vital for clear vision at different viewing distances and is an example of the eye's remarkable ability to adapt to changing visual requirements.

The contraction of the ciliary muscle is a unique mechanism in the eye that actually reduces tension on the lens. This process is crucial for adjusting the shape of the lens and, consequently, for changing the eye's focal length to focus on objects at different distances. This phenomenon is known as **accommodation** and is necessary for clear vision at various distances. Here's how the contraction of the ciliary muscle reduces tension on the lens: 1. **Lens Structure:** The lens of the eye is a transparent, flexible, and elastic structure located behind the iris. Its main function is to bend (refract) incoming light rays so that they converge onto the retina, forming a focused image. 2. **Lens Shape and Tension:** The shape of the lens is determined by the tension exerted on it by the suspensory ligaments (zonular fibers) that attach to the lens capsule. When the ciliary muscle is at rest, it exerts tension on the suspensory ligaments, which in turn flatten the lens. This is the default position for the lens when focusing on distant objects. 3. **Contraction of the Ciliary Muscle:** When you shift your focus from a distant object to a near one, the ciliary muscle contracts. This contraction has two important effects: - **Relaxation of the Suspensory Ligaments:** The ciliary muscle, when it contracts, relaxes or loosens the tension on the suspensory ligaments. This relaxation allows the lens to regain its natural, more rounded shape. - **Increase in Lens Curvature:** As the suspensory ligaments relax, the lens becomes more curved or bulged in shape. This increased curvature increases the refractive power of the lens, allowing it to bend light rays more strongly. This is necessary to focus on closer objects. 4. **Accommodation:** The changes in lens shape and tension induced by ciliary muscle contraction and relaxation are collectively known as accommodation. Accommodation allows the eye to adjust its focus from distant objects (flatter lens) to near objects (more rounded lens) by changing the curvature of the lens without the need for external devices like glasses. In summary, the contraction of the ciliary muscle reduces tension on the lens by relaxing the suspensory ligaments. This, in turn, allows the lens to adopt a more rounded shape, increasing its refractive power and enabling the eye to focus on objects at closer distances. This mechanism is vital for clear vision at different viewing distances and is an example of the eye's remarkable ability to adapt to changing visual requirements.