110 THE INDIVIDUAL ORGANISM 



Smell. The cell bodies of the olfactory nerves are located in the 

 mucous membrane that lines the upper parts of the nasal chambers; their 

 axons extend thence to the lower part of the cerebrum. Stimulation of 

 these neurons by volatile substances causes them to produce and transmit 

 impulses that are perceived as the sensation of smell. 



Taste. Unlike those of the olfactory nerves, the cell bodies of the 

 taste nerves are situated far from the point of stimulation, in the ganglia 

 of three cranial nerves. Fibers from these neurons extend to the tongue, 

 where they make contact with receptor cells located in taste buds. Stimu- 

 lation of the receptors by various dissolved substances gives rise to 

 impulses that are perceived as the four sensations sweet, sour, salty, and 

 bitter. The receptors are so localized that the tip of the tongue is most 

 sensitive to sweet; the sides, to sour; the sides and tip, to salty; and the 

 back, to bitter. The senses of smell and taste are intimately associated; 

 both respond to chemical stimulation, and a large part of what we think 

 of as the taste of substances is in reality their smell. 



Sight. The cell bodies of the optic nerves are located in the retina, 

 which forms the innermost layer of the eyeball and extends from the 

 point where the optic nerve enters the eye to the region of the lens. Out- 

 side the retina is a black layer, the choroid coat, containing many blood 

 vessels and continuous with the iris that surrounds the pupil. The iris 

 is furnished with muscles that regulate the size of the pupil and thus 

 govern the amount of light that reaches the retina. The outermost layer 

 of the eyeball is the hard and semirigid sclerotic coat. Anteriorly, the 

 sclerotic coat is continuous with the transparent cornea. The lens is 

 located just back of the iris and is held in place by ligaments. These 

 ligaments divide the interior cavity of the eyeball into two compartments 

 — a smaller outer space between the lens and the cornea, filled with a 

 watery fluid (the aqueous humor), and a larger chamber between the lens 

 and the retina, filled with a more viscid substance (the vitreous humor). 

 Light entering the eye is refracted by the spherically curved cornea and 

 also by the lens, so that images are sharply focused on the retina. The 

 position and degree of convexity of the lens are regulated by the involun- 

 tary ciliary muscles, which extend forward from the region of the ligaments 

 of the lens. The ligaments are normally under tension and hold the lens 

 in a flattened shape adapted to distant vision. When the ciliary muscles 

 contract, they relieve the tension on the lens ligaments, and the elastic 

 lens takes on a more rounded shape adapted to close vision. With increas- 

 ing age the lens stiffens, resulting in loss of accommodation. 



The light receptor cells of the eye are the rods and cones. Peculiarly 

 enough they are not situated on the side of the retina exposed to the light, 

 but next to the choroid coat under layers of nerve fibers, blood vessels, 

 and connective tissue, and their sensitive ends are turned away from the 

 light. Each receptor is composed of an inner segment much like an ordi- 



