(a) List the principal sense organs in mammals and their functions. (b) (i) Describe the eye defect in which only distant objects are seen clearly. (ii) How...

(a) Principal sense organs in mammals and their functions

(b)(i) The defect in which only distant objects are seen clearly

This defect is long sight (hypermetropia / far-sightedness). The person can see distant objects clearly but near objects appear blurred. It is caused by the eyeball being too short from front to back (or by the lens being too weak/flat and the ciliary muscles having lost their elasticity), so that light rays from a near object are brought to a focus behind the retina instead of exactly on it.

(b)(ii) Correction

Long sight is corrected by wearing spectacles fitted with a convex (converging) lens. The convex lens converges (bends inward) the diverging rays from a near object before they enter the eye, shortening the effective focal length so that the eye then brings them to a focus exactly on the retina, and near objects are seen clearly. The two diagrams below show the uncorrected eye (rays focusing behind the retina) and the corrected eye (a convex lens placed in front bringing the rays to a focus on the retina).

(c) Vertical section through the mammalian skin

The skin has two main layers: an outer epidermis (with a dead, cornified surface layer, a granular layer, and a living Malpighian / germinative layer that makes new cells and the pigment melanin) and, below it, the thicker dermis. The dermis contains the hair growing from its hair follicle, the erector (hair) muscle, the sebaceous (oil) gland, the coiled sweat gland opening at a sweat pore, sensory nerve endings, and blood capillaries. Beneath the dermis lies the subcutaneous fat (adipose) layer which stores fat and insulates the body. A large labelled vertical section is shown below.