08: The Nervous System : Icse-x-biology

with Solutions -

Note: Please signup/signin free to get personalized experience.

10 minutes can boost your percentage by 10%

Note: Please signup/signin free to get personalized experience.

#
ICSE
Answers for Chapter 8 The Nervous System Class 10 Selina Biology
Exercise 1

Qstn #1
Describe the mechanism of focusing the image of a distant object in your eye when you raise your head after reading a book.
Ans : While reading a book, the lens is more convex or rounded due to contraction of ciliary muscles because the book is usually read from a short distance. When we raise our head and look at a distant object, the ciliary muscles relax to build the tension on the suspensory ligament so that they can stretch the lens. This change in the curvature of the lens makes us focus on distant object.

Qstn #2
Sometimes you remember a vivid picture of a dream you saw. What is the role of your eyes in this experience?
Ans : The brain sees the vivid picture of the dream through the eyes. Our eyes have actually never seen the vivid picture. This is an example of optical illusion. The area of dream is controlled by the cerebrum of the central nervous system. So sometime we can remember the vivid picture seen in the dream.

Qstn #3
By closing the eyes and gently pressing them by your palms, you may see some specks of brilliant light. How do you get this sensation while there is no light entering your eyes?
Ans : If we look at a bright object and then close our eyes, the sensation of light persists for a short period. This is known as persistence image or the after image. It lasts for one-tenth of a second. Therefore by closing the eyes and gently pressing them with your palms, you see some specs of brilliant light.

Qstn #4
Explain the terms 'adaptation' and accommodation' with reference to the eye.
Ans : Adaptation is the ability to adjust vision in bright and dark areas. When we enter a dark room from bright light, the rhodopsin pigment broken down in bright light is regenerated. It dilates the pupil and allows more light to enter the eyes. This is called dark adaptation. On the other hand, if we enter bright area from a dark room, the rhodopsin pigment is bleached. This constricts the pupil and reduces the light entering the eyes. This is called 'light adaptation' Accommodation is the process of focusing the eye at different distances. This is mainly brought about by a change in the curvature of the lens. When the ciliary muscles contract, the lens becomes thicker and we are able to focus a nearby object. On the other hand when the ciliary muscles relax, the lens remains stretched i.e. the normal condition and we are able to focus on distant object.

Qstn #5
You do not enjoy watching a movie from a very short distance from the screen in a cinema hall. Why?
Ans : Our eyes are designed to focus at a great variety of distances. To focus constantly at a short distance can make the lens focusing muscles fatigued. Therefore, we do not enjoy watching a movie from a very short distance from the screen in cinema hall.

Enumerate the common defects of vision, their causes and the possible methods of correcting them.

Ans :

Defect of vision	Cause	Corrective Measure
Myopia	Lengthening of eye ball from front to back or the lens is too curved.	Using suitable concave lens.
Hyperopia	Shortening of eye ball from front to back or the lens is too flat.	Using suitable convex lens.
Astigmatism	Uneven curvature of the cornea.	Using suitable cylindrical lenses.
Presbyopia	Loss of flexibility of lens.	Using suitable convex lens.
Cataract	Lens turning opaque	Surgery or use of convex lens or implantation of plastic lens.
Colour Blindness	Genetic defect	No control measure
Squint	Formation of cross-eye	Surgery and suitable exercise.

Qstn #7
Name the three ear ossicles. How do they contribute in the mechanism of hearing?
Ans : The three ear ossicles are: Malleus (hammer), Incus (anvil) and Stapes (stirr up).
The last ear ossicle, stapes, vibrates and transmits the vibration to the oval window.
The role of other two ear ossicles is to magnify the vibration of stapes as a result of their lever like action.

Qstn #8
What is meant by power of accommodation of the eye? Name the muscles of the eye responsible for the same ................
Ans : The process of focusing the eye at different distances is called the power of accommodation. The ciliary muscles are responsible for the power of accommodation.E. Structured/Application/Skill Type

Qstn #1
With reference to the functioning of the eye, answer the questions that follow:

#1-a
What is meant by power of accommodation of the eye?
Ans : The ability of the eye to focus sharply on things which are near to the eye as well as far off is known as the power of accommodation.

#1-b
What is the shape of the lens during (1) near vision (2) distant vision?
Ans : Shape of the eye:
• Near vision - flattened
• Distant - rounded or more convex

#1-c
Name the two structure in the eye responsible for bringing about the change in the shape of the lens.
Ans : Ciliary muscles and suspensory ligament