This chapter describes the organs that sense touch, pressure, pain and temperature as well as the special senses of smell, taste, sight, hearing and balance.
After completing this section, you should know:
- that the general senses of touch, pressure, pain etc are situated in the dermis of the skin and in the body
- that the special senses include those of smell, taste, sight, hearing, and balance
- the main structures of the eye and their functions
- the route taken by light through the eye to the retina
- the role of the rods and cones in the retina
- the advantages of binocular vision
- the main structures of the ear and their functions
- the route taken by sound waves through the ear to the cochlea
- the role of the vestibular organ (semicircular canals and otolith organ) in maintaining balance and posture
- Receptors for touch, pressure, pain and temperature are found in the skin. Receptors in the muscles, tendons and joints inform the brain of limb position.
- The olfactory organ in the nose responds to chemicals in the air i.e. smell.
- Taste buds on the tongue respond to a limited range of chemicals dissolved in saliva.
- The eyes are the organs of sight. Spherical eyeballs situated in orbits in the skull have walls composed of 3 layers.
- The tough outer sclera protects and holds the shape of the eyeball. At the front it becomes visible as the white of the eye and the transparent cornea that allows light to enter the eye.
- The middle layer is the choroid. It most animals it absorbs stray light rays but in nocturnal animal it is reflective to conserve light. At the front of the eye it becomes the iris with muscles to control the size of the pupil and hence the amount of light entering the eye.
- The inner layer is the retina containing the light receptor cells: the rods for black and white vision in dim light and the cones for colour and detailed vision. Nerve impulses generated by these cells leave the eye for the brain via the optic nerve.
- The lens (with the cornea) helps focus the light rays on the retina. Muscles alter the shape of the lens to allow near and far objects to be focussed.
- Aqueous humour fills the space immediately behind the cornea and keeps it in shape and vitreous humour, a transparent jelly-like substance, fills the space behind the lens allowing light rays to pass through to the retina.
- The ear is the organ of hearing and balance.
- The external pinna helps funnel sound waves into the ear and locate the direction of the sound. The sound waves travel down the external ear canal to the eardrum or tympanic membrane causing it to vibrate. This vibration is transferred to the auditory ossicles of the middle ear which themselves transfer it to the inner ear. Here receptors in the cochlea respond by generating nerve impulses that travel to the brain via the auditory (acoustic) nerve.
- The Eustachian tube connects the middle ear with the pharynx to equalise air pressure on either side of the tympanic membrane.
- The vestibular organ of the inner ear is concerned with maintaining balance and posture. It consists of the semicircular canals and the otolith organs.