(a) What is a sensory cell?
(b) State one function each for the following parts of mammalian eye: (i) lens; (ii) vitreous humour; (iii) optic nerve.
(c) (i) List four structures which protect the eye from injury.
(ii)Explain briefly how each of these structures performs its function.
(d) Explain why a person in a dark room is dazzled for some seconds when he is suddenly exposed to bright sunlight.
Explanation
(a) Sensory cells are specialised cells which receive a particular stimulus. Normally they convert stimuli into impulses and transmit such impulses or sensation to the brain or spinal cord or central nervous system.
(b) (i) The function of lens is that it reflects rays or makes fine adjustment to focus object on the retina
(ii) Vitreous humour reflects light rays onto the retina or helps maintain spherical shape of the eye
(iii) Optic nerve transmits sensory impulses to the brain or transmits sensory impulses from the brain
(c )(i) The four structures which protect the eye against injury are
(1) Eye lids
(2) Eye lashes
(3) Eye socket or bonying socket
(4) conjunctive.
The eye lid is closed to protect the eye from foreign particles or mechanical injury. During blinking it carries water from the tear glands to moisten or irrigate the eye with tear or water to keep it moist. Eye lashes also acts as a hood and protects the eye from glare or excessive light. It also shields the eye against sweat, water or dust. Conjunctiva provides protective shield to the inner most delicate structure and gets inflamed during infection instead of the entire eye. Eye socket houses or encloses the eye balls. It shields or gives protection from mechanical injury, e.g. from a blow.
(d) In a dark room, the iris contracts while the pupil dilates or opens widely to allow in more light. On sudden exposure to bright sun-light, the pupil which is still wide open allows in too much light which bleaches pigment in the rod cells and this causes the dazzling sensation experienced. The iris muscle gradually relaxes within a few seconds. The pupil constricts gradually to regulate the entry of the required amount of light for the vision to return to normal.