Accommodation and sight defects of the eye, the skin and the ear

Outline how animals see objects in the environment

1. Light rays from an object travel in a straight line to the eyeballs

2. The light rays pass through the structure at the front of the eyeball, through the pupil and then focuses on the retina

3. The light sensitive cells of the retina are stimulated by the light rays.

4. Impulses are sent to the brain by the light sensitive cells of the retina along the optic nerve.

5. The brain then forms an image of size, shape, color and distance away from the object.

Define the term accommodation

This refers to the adjustment of the lens for a focusing on near and distant objects.

What happens when the eye focuses on distant objects?

The ciliary muscles )the circular ring of muscle fibers which alter the shape of the lens during accommodation) relax. This pulls the suspensory ligaments tight resulting in the lens being flattened (less convex). This shape refracts the light less which results in the image being sharply focused on the retina.

What happens when the eye focuses on near objects?

The ciliary muscles contract which reduces the tension on the suspensory ligaments resulting in them being slackened. As a result, the ligaments pull less on the lens causing a more rounded (more convex) shape. A very curved shape refracts the light more which results in the image being focused sharply on the retina.

How does light affect the size of the pupil?

When the pupil is open widely, more light enters the eye than when the pupil is small. In bright lights the pupil contracts to protect the eye from excess light while in dim light it expands to take advantage of as much light as possible. To expand the pupil the circular muscles in the iris relax and the radial muscles contract, pulling the iris back and so open in the pupil. In contrast to make the pupil smaller the radial muscles relax and the circular muscles contract.

What are the 5 sight defects?

1. Hypermetropia (long sightedness)

2. Myopia (short sightedness)

3. Astigmatism

4. Cataract

5. Glaucoma

What is hypermetropia (long-sightedness)

This is a sight defect caused by the eyeball being too short from front to back or the lens being too flat which results in light from distant objects being focused on the retina while light from near objects focus behind the retina.

How can hypermetropia be corrected?

This can be corrected by wearing convex or converging lenses

What is myopia?

This is a sight defect that is caused by the eyeball being too long from front to back or the lens being too curved. As a result light rays from a distant object are bent more than necessary and focus in front of the retina while light rays from near objects focus on the retina resulting in near objects being seen more clearly than distant ones.

How can myopia be corrected?

This can be corrected by wearing concave or diverging lens which helps the person to see far objects clearly.

What is astigmatism and how can it be corrected?

This is a sight defect which occurs when the surface of the lens or cornea is curved irregularly. Specially shaped lenses, which balance out these irregularities, can be worn to provide a clear image on the retina.

What is Cataract and how can it be corrected?

This is a sight defect which occurs when the lens become opaque and light cannot pass through. This can be corrected by removing the lens during surgery, making adjustments to the vision with appropriate spectacles or contact lenses or replacing the lens with an intraocular lens.

What is glaucoma and how can it be corrected?

This is a condition which occurs when there is a buildup of pressure in the aqueous humor which can affect the optic nerve. It is early stages this can be corrected with medications such as eye drops and oral medications while in its later stages surgery may be necessary to ease the pressure in the eyeballs. If left untreated it will result in blindness.

What are the two functions of the ear?

Hearing and Balance

Outline how animals hear sounds from the environment

1. The sound waves reach the ear and is directed by the pinna (outer ear) into the auditory canal.

2. The sound waves travel down the auditory canal (ear canal) to the eardrum (tympanic membrane).

3. The eardrum vibrates when hit by the sound waves.

4. The ear bones (ear ossicles) in the middle ear vibrate.

5. The vibrations at the oval window start pressure waves in the cochlear fluid (a section of the inner ear).

6. The hair cells (receptors) in the cochlea vibrate in response to the pressure waves in the cochlear fluid.

7. Nerve impulses generated by the hair cells travel along the auditory nerve to the brain and we hear.

8. The vibrations then pass away to the round window and we are ready to hear again.

What is the tympanic membrane (tympanum)?

This is a thin membrane which is pulled taut and separates the outer and the middle ear

Explain the role of the tympanum in hearing

When hit by sound waves the tympanic membrane converts them into movement which are then amplified as they pass through the ossicles. Pressure on both sides of the membrane must be equal to ensure that it stays straight and taut so that messages can pass on efficiently.

Why does the sensation of the ear ‘popping’ occur?

This occurs when the tympanic membrane moves back into position when the pressure on both sides equalizes.

What are the three regions of the ear?

1. The outer ear

2. The middle ear

3. The inner ear

What makes up the inner ear?

The cochlea and the vestibular apparatus

What is the cochlea?

This is a large, coiled, three chambered tube which is responsible for our sense of hearing.

What is the vestibular apparatus?

This is the structure responsible for our sense of balance and information about the position and movement of our body. It is made up of two parts:

1. The semicircular canals which detect movement of our head

2. The utricle and the saccule (ear sac) which detect the position of the head.

Explain how the ear is associated with balance

The vestibular apparatus is the structure in the inner ear responsible for the sense of balance. It is made up of the semicircular canals, which detect the movement of the head and therefore the body, and the utricle and saccule which detect the position of the head and therefore the body. Hair cells (receptors) present in these structures deflect on movement causing impulses to be sent to the brain.

what are the semicircular canals?

These are three fluid-filled tubes arranged at right angles to each other, in the three planes, detecting any movement of the head, and therefore the body.

What movements are detected by the utricle?

UV lights

This responds to the vertical movements of the head

What movements are detected by the saccule?

This responds to the lateral (sideway) movements of the head.

What is the skin?

The is the largest specialized organ of the body which encloses the muscles and other internal organs.

What are the three layers of the skin?

The three layers of the skin are:

1. The epidermis which is the outermost layer.

2. The dermis which is below the epidermis and contains connective tissue, blood vessels, and nerve endings.

3. The hypodermis (subcutaneous layer) which is the bottom layer made up mostly of fat cells.

Define the term homeotherm

This is an organism (Birds and Mammals) that can maintain its body temperature regardless of the external environmental conditions. This occurs through metabolic processes such as respiration and the constriction of blood vessels.

Define the term poikilotherm

This is an organism (Fish, reptiles, amphibians, and invertebrates) that is unable to regulate its body temperature by psychological means. They instead rely on external environmental conditions to keep their body warm.

What are 5 examples of homeotherms

Examples include humans, dogs, mice, whale, and pigeon.

What are 5 examples of poikilotherms

Examples include lizards, frogs, snakes, sea turtles, and insects.

Explain how the human body temperature is regulated by the skin

The human body regulates temperature through the skin by processes such as sweating, vasodilation, vasoconstriction and the relaxation and constriction of the hair erector muscles to maintain homeostasis and thermoregulation, allowing the body to lose or retain heat as needed.

How does the body lose heat?

This occurs through three main processes:

1. Increase in sweating- Heat is removed from the body when the sweat evaporates.

2. Vasodilation- this occurs when the capillaries in the dermis dilate increasing blood flow through the skin which facilitates heat loss from the blood

3. The relaxation of the hair erector muscles- The hairs lie flat allowing moving air to get closer to the skin and remove heat.

How does the body conserve heat?

1. Sweating decreases

2. Vasoconstriction occurs- the capillaries in the dermis constrict decreasing blood flow to the skin which facilitates heat retention in blood vessels deep in the body.

3. The contraction of the hair erector muscles- The hairs stand up trapping a layer of warm air next to the skin (insulation)

Describe temperature regulation in birds

In cold temperature the hair erector muscles contract and the bird’s feathers stand out from the skin trapping air close to the skin and acting as an insulator. It also stops air flow over the skin reducing loss of heat by convection.